I've used Windows 7 on my Aspire One and 64-bit on my Macbook. I've had no problems with either machine (although I've since reverted back to XP on the Aspire One), and I'll probably be grabbing a copy when it comes out in October. I've seen it once and wasn't impressed. It seems like "Ubuntu for people who can't figure out Ubuntu" and from what I've read it severely taxes system resources. It sure was pretty, though.
I installed Win7 on two different NetBooks so far, and judging by the small hardware those things have, it runns quite nice and usable. Might really make it to my main OS once it's finished.
I use Ubuntu 9.04 frequently on my laptop, along with Windows 7. I would use Ubuntu on my desktop, but it keeps crashing during install. I think the problem is my DVD drive though. It's been on it's last leg for about a year and a half now. =P