My use case is to retrieve the list of all email alerts in my org with their name, description, sender-address, recipients, & CC'ed recipients. For this I have come up with the below code which I execute from the anonymous exec in dev console.

<pre>
MetadataService.MetadataPort service = MetadataService.createService();

List<MetadataService.ListMetadataQuery> queries = new List<MetadataService.ListMetadataQuery>();
MetadataService.ListMetadataQuery queryEmailAlert = new MetadataService.ListMetadataQuery();
queryEmailAlert.type_x = 'WorkflowAlert';
queries.add(queryEmailAlert);

//get metadatalist
MetadataService.FileProperties[] properties = service.ListMetaData(queries,30);
for(MetadataService.FileProperties emailalert:properties){
        system.debug('Email Alert Name:::'+emailalert.fullname);

       //read metadata
       MetadataService.Metadata[] mdInfo = service.readMetadata('WorkflowAlert',new String[]{emailalert.fullname}).records;
        for(MetadataService.Metadata md:mdInfo){
            MetadataService.WorkflowAlert alert = (MetadataService.WorkflowAlert)md;
            system.debug('description:::'+alert.description);
            system.debug('From:::'+alert.senderAddress);
        }
}
</pre>

But the execution stops with the error:
<pre>
"incompatible types since an instance of MetadataService.Metadata is never an instance of MetadataService.WorkflowAlert"
</pre>

I believe it doesn't allow Metadata to be Typecast to WorkflowAlert. But I am not sure if the results can be achieved without doing that.
Can you please help letting me know where I am going wrong here?
Greatly appreciate your help! Many Thanks!

Note:
Since the class generated by the MetadataAPI wsdl exceeds my Apex Limit, I have cut it short to contain only what is required to achieve the required use case. Below is my short version of the MetadataService class used here for additional info, in case you like to have a look.

<pre>
//Generated by wsdl2apex
public class MetadataService {
    public class MetadataPort {
        public String endpoint_x = 'https://cs30.salesforce.com/services/Soap/m/30.0';
        public Map<String,String> inputHttpHeaders_x;
        public Map<String,String> outputHttpHeaders_x;
        public String clientCertName_x;
        public String clientCert_x;
        public String clientCertPasswd_x;
        public Integer timeout_x;
        public MetadataService.SessionHeader_element SessionHeader;
        public MetadataService.DebuggingInfo_element DebuggingInfo;
        public MetadataService.CallOptions_element CallOptions;
        public MetadataService.DebuggingHeader_element DebuggingHeader;
        private String SessionHeader_hns = 'SessionHeader=http://soap.sforce.com/2006/04/metadata';
        private String DebuggingInfo_hns = 'DebuggingInfo=http://soap.sforce.com/2006/04/metadata';
        private String CallOptions_hns = 'CallOptions=http://soap.sforce.com/2006/04/metadata';
        private String DebuggingHeader_hns = 'DebuggingHeader=http://soap.sforce.com/2006/04/metadata';
        private String[] ns_map_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata', 'MetadataService'};
        public MetadataService.FileProperties[] listMetadata(MetadataService.ListMetadataQuery[] queries,Double asOfVersion) {
            MetadataService.listMetadata_element request_x = new MetadataService.listMetadata_element();
            MetadataService.listMetadataResponse_element response_x;
            request_x.queries = queries;
            request_x.asOfVersion = asOfVersion;
            Map<String, MetadataService.listMetadataResponse_element> response_map_x = new Map<String, MetadataService.listMetadataResponse_element>();
            response_map_x.put('response_x', response_x);
            WebServiceCallout.invoke(
              this,
              request_x,
              response_map_x,
              new String[]{endpoint_x,
              '',
              'http://soap.sforce.com/2006/04/metadata',
              'listMetadata',
              'http://soap.sforce.com/2006/04/metadata',
              'listMetadataResponse',
              'MetadataService.listMetadataResponse_element'}
            );
            response_x = response_map_x.get('response_x');
            return response_x.result;
        }
        public MetadataService.ReadResult readMetadata(String type_x,String[] fullNames) {
            MetadataService.readMetadata_element request_x = new MetadataService.readMetadata_element();
            MetadataService.readMetadataResponse_element response_x;
            request_x.type_x = type_x;
            request_x.fullNames = fullNames;
            Map<String, MetadataService.readMetadataResponse_element> response_map_x = new Map<String, MetadataService.readMetadataResponse_element>();
            response_map_x.put('response_x', response_x);
            WebServiceCallout.invoke(
              this,
              request_x,
              response_map_x,
              new String[]{endpoint_x,
              '',
              'http://soap.sforce.com/2006/04/metadata',
              'readMetadata',
              'http://soap.sforce.com/2006/04/metadata',
              'readMetadataResponse',
              'MetadataService.readMetadataResponse_element'}
            );
            response_x = response_map_x.get('response_x');
            return response_x.result;
        }
    }
    public class ReadResult {
        public MetadataService.Metadata[] records;
        private String[] records_type_info = new String[]{'records','http://soap.sforce.com/2006/04/metadata',null,'0','-1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'records'};
    }
    public class Metadata {
        public String fullName;
        private String[] fullName_type_info = new String[]{'fullName','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'fullName'};
    }
    public class WorkflowEmailRecipient {
        public String field;
        public String recipient;
        public String type_x;
        private String[] field_type_info = new String[]{'field','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] recipient_type_info = new String[]{'recipient','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] type_x_type_info = new String[]{'type','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'field','recipient','type_x'};
    }
    public class WorkflowAlert {
        public String[] ccEmails;
        public String description;
        public Boolean protected_x;
        public MetadataService.WorkflowEmailRecipient[] recipients;
        public String senderAddress;
        public String senderType;
        public String template;
        private String[] ccEmails_type_info = new String[]{'ccEmails','http://soap.sforce.com/2006/04/metadata',null,'0','-1','false'};
        private String[] description_type_info = new String[]{'description','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] protected_x_type_info = new String[]{'protected','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] recipients_type_info = new String[]{'recipients','http://soap.sforce.com/2006/04/metadata',null,'0','-1','false'};
        private String[] senderAddress_type_info = new String[]{'senderAddress','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] senderType_type_info = new String[]{'senderType','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] template_type_info = new String[]{'template','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'ccEmails','description','protected_x','recipients','senderAddress','senderType','template'};
    }
    public class DebuggingInfo_element {
        public String debugLog;
        private String[] debugLog_type_info = new String[]{'debugLog','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'debugLog'};
    }
    public class CallOptions_element {
        public String client;
        private String[] client_type_info = new String[]{'client','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'client'};
    }
    public class DebuggingHeader_element {
        public MetadataService.LogInfo[] categories;
        public String debugLevel;
        private String[] categories_type_info = new String[]{'categories','http://soap.sforce.com/2006/04/metadata',null,'0','-1','false'};
        private String[] debugLevel_type_info = new String[]{'debugLevel','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'categories','debugLevel'};
    }
    public class SessionHeader_element {
        public String sessionId;
        private String[] sessionId_type_info = new String[]{'sessionId','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'sessionId'};
    }
    public class LogInfo {
        public String category;
        public String level;
        private String[] category_type_info = new String[]{'category','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] level_type_info = new String[]{'level','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'category','level'};
    }
    public class listMetadataResponse_element {
        public MetadataService.FileProperties[] result;
        private String[] result_type_info = new String[]{'result','http://soap.sforce.com/2006/04/metadata',null,'0','-1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'result'};
    }
    public class ListMetadataQuery {
        public String folder;
        public String type_x;
        private String[] folder_type_info = new String[]{'folder','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] type_x_type_info = new String[]{'type','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'folder','type_x'};
    }
    public class FileProperties {
        public String createdById;
        public String createdByName;
        public DateTime createdDate;
        public String fileName;
        public String fullName;
        public String id;
        public String lastModifiedById;
        public String lastModifiedByName;
        public DateTime lastModifiedDate;
        public String manageableState;
        public String namespacePrefix;
        public String type_x;
        private String[] createdById_type_info = new String[]{'createdById','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] createdByName_type_info = new String[]{'createdByName','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] createdDate_type_info = new String[]{'createdDate','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] fileName_type_info = new String[]{'fileName','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] fullName_type_info = new String[]{'fullName','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] id_type_info = new String[]{'id','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] lastModifiedById_type_info = new String[]{'lastModifiedById','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] lastModifiedByName_type_info = new String[]{'lastModifiedByName','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] lastModifiedDate_type_info = new String[]{'lastModifiedDate','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] manageableState_type_info = new String[]{'manageableState','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] namespacePrefix_type_info = new String[]{'namespacePrefix','http://soap.sforce.com/2006/04/metadata',null,'0','1','false'};
        private String[] type_x_type_info = new String[]{'type','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'createdById','createdByName','createdDate','fileName','fullName','id','lastModifiedById','lastModifiedByName','lastModifiedDate','manageableState','namespacePrefix','type_x'};
    }
    public class listMetadata_element {
        public MetadataService.ListMetadataQuery[] queries;
        public Double asOfVersion;
        private String[] queries_type_info = new String[]{'queries','http://soap.sforce.com/2006/04/metadata',null,'0','-1','false'};
        private String[] asOfVersion_type_info = new String[]{'asOfVersion','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'queries','asOfVersion'};
    }
    public class readMetadata_element {
        public String type_x;
        public String[] fullNames;
        private String[] type_x_type_info = new String[]{'type','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] fullNames_type_info = new String[]{'fullNames','http://soap.sforce.com/2006/04/metadata',null,'0','-1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'type_x','fullNames'};
    }
    public class readMetadataResponse_element {
        public MetadataService.ReadResult result;
        private String[] result_type_info = new String[]{'result','http://soap.sforce.com/2006/04/metadata',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://soap.sforce.com/2006/04/metadata','true','false'};
        private String[] field_order_type_info = new String[]{'result'};
    }
    public static MetadataService.MetadataPort createService() {
        partnerSoapSforceCom.Soap sp = new partnerSoapSforceCom.Soap();
//        system.debug('loging in...');
        partnerSoapSforceCom.LoginResult loginResult = sp.login('uid','pwd');
//        system.debug('Login Result:::' +loginResult);

        MetadataService.MetadataPort service = new MetadataService.MetadataPort();
        service.SessionHeader = new MetadataService.SessionHeader_element();
        service.SessionHeader.sessionId = loginResult.sessionId;
//      service.SessionHeader.sessionId = UserInfo.getSessionId();
        return service;   
    }
}
</pre>