So far I've only had to run it on my compatibility-test systems -- it's not my main working/development OS and never will be. I enjoy Mac OSX a lot, I work happily with Linux, and I even find XP pleasantly productive much of the time.
The times I've had to work on Vista, I feel like I'm in one of those bad dreams where you're running through thick mud and can't make any headway, and there is evil all around.
I believe that my LARGE research institution will not adopt Vista at all, and we buy hundreds of computers a year... and I suspect I will NEVER have to use any form of vista at all!!!
Linux, OSX... all good! XP - OK, but time consuming to assure security.