In short, if you held off on upgrading to Vista, it’s paid off.
If you’re happy on XP, there’s really no pressure to leave.
That’s about where I’d put it. I’m still cross-grading over to Mac whenever my current XP setup falls over (it’s doing fine tho’)
There’s a set of links at the end of the article for those that have already made the Vista plunge. I can’t speak for their reliability and if you’re on Vista you’ve probably already chased down a lot of that info but it seems like a handy bookmark.