I used Windows XP for years and then switched to Vista about six months ago. AFAICT, there is nothing slower about Vista than XP on the same machine (an old ThinkPad T60). But, because of usability improvements in Vista, I can get things done faster. UAC removes a lot of the hassles of running as a limited user in XP, and that is the main reason I upgraded. UAC probably saves me at least an hour a week.
