Everybody I have spoken to who has got Vista finds that something, sometimes something important, doesn't work under it.
And - don't forget - a PC is no more than a machine for running applications, if you can't run the application(s) then why have it?
I would stick with XP.
At work, I stick with win2000 because everything I have works fine under it and a lot of software is no longer supported. And we run some mission-critical PCs under NT4. Some test gear, believe it or now, runs under win3.1...