IntelliSense fixes by itself would be enough reason to upgrade
- the blog entry I linked above does sound promising. But from reading the vcblog, it does seem like most 2008 enhancements are for dotNET. I ended up on one of those link-following cruises, and it's pitiful that intrinsics are still pretty lousy; yeah, they added support for new instructions, but there's still code generation problems. Might as well write external assembly and link that in, at least you know what you're going to get.
Also a shame that, seemingly, IDE as well as compilers are still exclusively 32bit. Sure, you can target
x64, but the tools are 32bit. I would be interested in seeing whether the additional registers of x64 could do something for compiler speed...
But all in all, if there's intellisense fixes and the IDE is a bit snappier, and no new significant bugs are introduced, it'll be worth it.