This is my personal experience.
I've been a PC user/gamer for about 25 years counting my C64 days.
World of Warcraft + Ventrilo = Vista's audio layer pegging CPU at 100%
Oblivion = Performance decrease unless Aero is turned off (one of the big selling points for vista)
EAX (creative audio standard that's been used for years) is not supported in Vista. That super fantabulous X-Fi card with 5.1 and EAX effects? Forget it. You get stereo.
The really handy "Windows has blocked some startup programs, click here to view them" window ALWAYS appears when you boot up. Can't select the programs/icons as "I know stop telling me".
UAC windows pops up when running a utility program. Requests permission to run. I like that. But why do I have to do it every single time I run my disk defragmenter? What, there's no option to flag this as a "permission granted always" program?
For an OS that's been in development for 5 years I was underwhelmed. 3 of the biggest suppliers of hardware addons to PC's didn't/don't have satisfactory drivers for Vista. Creative, ATI, Nvidia. The performance is laggy. Aero is pretty but a resource hog. Irritating popups. Absolutely no reason to upgrade from Windows XP in the foreseeable future. It's unfortunate that I did last month to try it out.
For the record my system has the following: 2gigs ram, 3.4G P4, ATI X1900XTX, X-FI, dual 10k rpm Raptors, etc.
That's my 2 cents