I like the Visual Studio 2005 Class Diagrammer
I looked at the Visual Studio 2005 Class Diagrammer briefly when I first started using the product. I didn't think it did much, thought Visio was better, and didn't use it anymore. End of story.
While preparing a presentation on my current project I found Visio wasn't creating the pretty diagrams I wanted. I decided to try the Diagrammer again. After you create a class diagram, you can hit the + key or hit the triple upside down ^ icon to get the diagram to expand. It does a nice job of displaying your class.
I've found a few useful features in the class diagram menu: Show Base Class, Show Derived Class, Change Members Format --> display full signature. There is also a Refactor -->Extract Interface which looks handy.
Changes made to the diagram or in code are instantly synchronized between the two.
Unfortunately there is no integration between Visio and the Class Diagrammer. You can click on an object in the class designer and copy paste it as a picture into PowerPoint Visio (I took independent pictures of each object and created new connector lines in Visio)