Stable, reliable, a little slow at times (but blame the user for that*)
Ran the RC2 build for a long time (until the last day it was officially allowed all over the world
), ran RTM build for few months and now in SP1 (private beta).
It's been pretty stable. Had some issues with sleep/hibernation earlier with my ThinkPad T60, but after I dumped my docking station, it's been good. performance is slow (* above), but that's mainly due to me running a ton of things. This is my dev machine, so I have VS, outlook, sidebar gadgets, AV, IE windows, perforce source control, couple of explorer windows, media player, etc all open at once. But it's not as bad as I saw XP behave (over-time).
One think I hope they fix in SP1 is the unneccessary usage of disk space (temp install files, left over stuff from WU or installs, system restores, etc).
Edit: oh and to add to others here, I also haven't seen any BSODs yet (even driver related, since most of my drivers were either already on disk or WU found them).