Here's another reason to install Visual Studio SP1 - it fixes one class of "intellisense stops working" problems. Intellisense stops working a lot for C++ developers. It doesn't happen very often for me, but people mention it quite often when I speak and some of my coworkers are less lucky than I am because it happens in their day to day work.
If Intellisense stops working for you and you're working on a C++ project, the workaround is to close Visual Studio, find and delete the .ncb file, and re-open Visual Studio. As you can imagine, that cuts into flow quite a bit. Now a KB article mentions that SP 1 fixes "In a Microsoft Visual C++ 2005-based solution that includes multiple projects, the IntelliSense of a project does not work correctly if the project references types from another Visual C++ or Visual C# project." Cool!