Vista builds on XP as XP built on Windows 2000. The only real issue that makes me squeamish is DirectX 10 and compatibility with DirectX 9 (or earlier) games.
I said the same thing about moving from Win2K to WinXP, but in the end some of the conveniences in how the XP shell handles folders full of pictures won me over. Besides, I'm a sucker for a pretty interface.
If and when I do get Vista, I'll be getting home premium. Gotta have that bloated but purdy Aero interface. I'm a sucker for a pretty interface. Anyway, although I have more than enough horsepower to run Vista, I have too many resource-hungry DirectX 9 games. Maybe in a year I'll think about it.