My primary machine at work has two partitions. One hosts WinXP, the other Windows Longhorn.
The cool thing about this randomness is that I haven't even booted into WinXP for about a week, and have been using Longhorn as my primary OS for the past three weeks. This includes development with Whidbey, Office-stuff, blog reading, surfing, etc., etc.
I'd be lying if I said I have had absolutely no problems, but considering the early state of the product and amount of functionality I've received as a result of the switch, I'm extremely happy. And I will come forth and say that I haven't had a single crash. Zero.
I'd cry if I didn't have my little analog clock and gray minibar any longer. Never looking back. ;)