I`ve been running Win 7 64 bit on my quadcore studio mainframe
Really really happy with it to be honest.
I`ve found it so stable I finally switched my Live PA Laptop over to Win 7 as well, and 32 gigs in I have had no hiccups (bar a problem with a firewire card getting pulled out during a set)
Some developers need to catch up and make more Win 7 functionality and exploit it more, but it is early days.
Ableton works great with it, as does all my mastering and authoring software.
Gonna go solid state for my system drives soon I think, eliminate moving parts!!!!
Disk defragmenting is a thing of the past with modern systems now.
So glad I finally dropped XP now