My opinion of Vista:
It works, but it has a few problems. Of course there is the ever present memory usage problem, but this isn't an issue for those who don't play games (I do though). It is more secure than XP - I haven't looked into just how much more secure it is though. With Vista MS is catching up to Mac and Linux in terms of what features and abilities its desktop has. I haven't seen any real benefits of DX10 yet though.
If I had not discovered Linux around spring of '07 I would probably be using Vista more often now, now that drivers, etc. have had time to mature. As a Linux user though, all of my non-gaming needs are taken care of by a free, flexible, powerful and open source OS - so the only thing I would need any version of Windows for is gaming, and right now XP outperforms Vista in that regard. So, I have little reason to use Vista for the moment.
We'll see if it becomes a more appealing choice as a gaming OS as time goes on. It could, but it also might run out of time and lose to Windows 7 (if that one is supposed to be as efficient as they say).
Or, they could finally port the source engine to opengl or linux...then I'd stay in linux virtually all the time 
