My Mylyn perspective. Nothing special about it though except that Mylyn really shines when it is hooked in a bug tracking/task management system like FogBugz or Bugzilla or whatever.
We use FogBugz with our clients. The attached screenshot shows a case that a client created and shows Troy and I going back and forth as we work through the task. FogBugz has allowed us to integrate clients fully into the development process with no additional management overhead. As you can imagine, clients love being involved this intimately and is a huge help to keeping everything on track.
FogBugzMylynFoglyn (Mylyn -> FogBUGZ connector