RetrieveWorkItemHistory.cs

// utilising RestSharp and Json.NET - get via Nuget.

private WorkItem GetWorkItem(string workItemIdentifier)
{
  var restClient = new RestClient("https://account.visualstudio.com/defaultcollection");
  restClient.Authenticator = new HttpBasicAuthenticator("username", "password");
  
  // code removed here
            
  var changes = GetWorkItemHistoryComments(restClient, workItemIdentifier);
  
  // code removed here
}           

private IEnumerable<WorkItemCommentChange> GetWorkItemHistoryComments(RestClient activeClient, string workItemIdentifier)
{
  var request = new RestRequest("/_apis/wit/workitems/{id}/updates");
  request.AddParameter("api-version", "1.0-preview");
  request.AddUrlSegment("id", workItemIdentifier);

  var response = activeClient.Execute(request);

  dynamic json = JObject.Parse(response.Content);

  var changes = new List<WorkItemCommentChange>();

  string revisionDate = string.Empty;
  string revisedBy = string.Empty;
  string comments = string.Empty;

  // each one of these represents a revision
  foreach(var val in json.value)
  {
    // a revision contains multiple fields
    foreach(var ufield in val.fields)
    {
      if (ufield.field.id == -4) // System.ChangedDate
      {
        revisionDate = ufield.updatedValue; // "02/01/2014 01:21:00" (MM/dd/yyyy and UTC)
      }

      if (ufield.field.id == 9) // System.ChangedBy
      {
        revisedBy = ufield.updatedValue; // "Simon Waight"
      }

      if(ufield.field.id == 54) // System.History
      {
        comments = ufield.updatedValue; // can be plain text or HTML
      }
    }
    // we only care about comments
    if(!string.IsNullOrEmpty(comments))
    {
      changes.Add(new WorkItemCommentChange 
                    { 
                      ChangeDate = DateTime.ParseExact(revisionDate, "MM/dd/yyyy HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal), 
                      ChangedBy = revisedBy, 
                      CommentText = comments 
                    });
      comments = revisedBy = revisionDate = string.Empty;
    }
  }
  return changes;
} 

public class WorkItemCommentChange
{
  public DateTime ChangeDate { get; set; }
  public string ChangedBy { get; set; }
  public string CommentText { get; set; }
}