I've been using Win7 on my laptop since the RC was released and I'm impressed so far. I won't be installing it on my desktop because there's too much software to reinstall now and again when the RTM arrives, but the laptop has very little on it.
This laptop is over two years old so the hardware is not bad but not cutting edge. It came with XP and I installed Vista about six months or more ago. I've never had any specific issues with Vista although my desktop system is fairly heavy-duty. This laptop ran Vista OK but it was significantly slower than XP on the same machine or Vista on my desktop. I did have Aero Glass enabled, which was probably a mistake.
As soon as I installed Win7 I could appreciate an increase in performance. I don't have Glass enabled, which would account for some of that, but startup and shutdown times have both improved, as has application startup, shutdown and responsiveness. VS used to take quite some time to exit on Vista but it's gone in a second or two on Win7.
The only real complaint I have had with Vista is that UAC is a pain sometimes. I don't mind confirming an app but Vista will, at times, tell me that it's going to ask for confirmation first, then actually ask for confirmation. UAC is a lot easier to get along with on Win7, although I can't say for sure that that doesn't come with a reduction is security. I'm sure there's plenty of literature on the topic.
The only real issue I've had so far is that VS has locked up a few times, so that's probably the RC at work there. Another reason not to put it on my desktop. Although I haven't used it much, I've also got VS 2010 Beta 1 on that laptop and it runs fine, although it is slower that VS 2008. I'l put that down to its being a beta for now.
I am, at this very moment, installing Win 7 x64 on my girlfriends laptop. I've assured her that it will run better than the Vista x86 that was on there before, if only because it can access all 4 GB of RAM.