Jerry Pournelle's opinion
From his website:
I will have a good bit to say about VISTA in the column this week (or possibly in the opening column for May). In essence I will be saying Do Not Buy Vista, and DO NOT buy a new machine just now because it will have VISTA. Wait for a Service Pack that has some DRIVERS. VISTA is just too darned much trouble for what you get. Wait for the updates. Keep your present machinery and stay with XP. More over at Chaos Manor Reviews next week or the week after.
My own opinion, although I don't know nearly as much about computers, is that it is death to buy a Windows OS that's less than two or three years old.