The May 2009 MSDN Magazine has a good article reviewing XML comments and their usefulness for code development and documentation. The article is primarily written for VB developers, but the concepts do apply to C# developers as well.
I've been using XML comments since Visual Studio 2005 came out. For your VB developers still developing legacy .NET 1.1 / VS 2003 apps, you can install the VBCommenter add-in, which will give you some of the basic functionality (C# developers have this out-of-the-box).
If you're not familiar with XML comments - or if you've heard of them, but haven't tried them out yet - give the article a look.