I primarily use Gentoo Linux, but I have Vista RC2, build 5744 installed on a partition on my PC. Though it is visually a huge improvement over Windows XP, I really am not very impressed with it. Admittedly I'm a Linux junkie and therefore biased, and I am running a beta (though build 5744 is supposed to be basically the RTM version of Vista :rolleyes2), it has been <i>very</i> unstable for me. I've seen more bluescreens than in five years of experience with XP. Not more than Windows 98 though.
It didn't have very good support for my hardware out of the box - I had to hack XP drivers for my sound card and wifi card in through device manager, instead of recognizing a chipset and using the generic driver.
Redmond made a lot of noise about "streamlining the experience for the user", but then they've stuck things like the sidebar in there, which just seems like extra bloat to me. They've also taken away useful parts of the OS by default, such as the menubar, and the "up one level" button (which I can't figure out how to get back, for the life of me). And it took control of my bootloader, which is a hassle to restore.

On the positive side though, DirectX10 looks <i>very</i> nice, and I look forward to playing some games in it, rather than through the Directx9.0c legacy chip (which seems a little laggy to me). The interface is a lot more attractive, which is good after working a lot with Beryl in Linux.
I also like the redesigned layout of the control panel; it's a lot more intuitive and things are where you'd expect them to be. At first it was a little frustrating with things not being where they used to be, but it was relatively quick to figure out the new place of most things.

On the whole, it's a nice improvement over XP, <i>if</i> the instability is fixed in the RTM release (which I haven't tried yet). I like that I can make most things how they are in XP, so I'm not totally jumping out of an environment into another that is completely different.