From Scott Hanselman's Computer Zen - Refector 5 Released - World Domination Assured:
Reflector 5 is out, and it is good. Go get it immediately, or just run your current version and go Help | Check for Updates.
Reflector is one of the most useful tools out there for understanding what's going on with an assembly and so everybody upgrade immediately. Scott has a pretty good list of the new features that Lutz has added in his post, the key ones (for me) being the code:// protocol handler (that's going to be really useful!), and the new functionality in the Analyze section...