I installed SP1 RC1 the first day it was available. Almost immediately my Vista box would hang overnight. It would seem to work great during the day but at night simply would not come out of sleep mode. So I uninstalled it.
This past weekend I had a few extra minutes (or was it hours?) so I paved the box and reinstalled Vista x64 deciding this time to not install any third party drivers that I didn't absolutely have to have. Turns out the only one I needed was for my Realtek audio chipset.
I reapplied SP1 RC1 and after 3 days no hangs. Everything seems rock solid. Well, it is an RC and Microsoft has an amazingly hard job getting all the different flavors of machines out there to play nice with Vista. All in all they do a fairly good job. SP1 RC1 stable for me.. so far.