James Bannan spent some time digging into a leaked version of an early build of Vista SP1, and has generally good things to say about it. He also mentions two compatibility and performance updates that Microsoft released "under the radar" (they are not on Windows or Microsoft Update yet), earlier this week.
I put both of these updates (KB938194 and KB938979) on my main Vista system earlier this week, and I am happy with the results. That said, I really wish that Microsoft would get it together with Vista. After it has been out nearly a year now, and there are still so many minor, but aggravating problems that people seem to run into.
Personally, I have had pretty good results with Vista, but I am still hesitant to recommend it to my parents (who are not that computer savvy). Part of the problem is that Windows XP so familiar to people, and it is really good enough for most people.