Seems pretty stable. Make sure you update all your plugs and RTAS instruments w/ latest versions. (BTW, N.I. Komplete 2 and Stylus RMX are working fine.) I have had problems with a few of the Sound Toys plugs (trying to instantiate Filter Freak crashes PT). They know that there are issues with their stuff on Quad G5's and are working on it.
You'll love how little time it takes to do file conversions and other processing. I am in constant"Tweak Head: mode now
Ryan Ulyate