Hi, I have some really serious problems with both the closed-source and open-source drivers.
Severe display corruption in both metacity and compiz. Usually triggered by playing games in wine.
X crashes - stops responding - while using gimp-devel (polygonal selection tool is insta-crash), openoffice writer (scrolling) and many games.
Compiz works rather well and using it actually prevents some of the crashes.
I'm cetain this isn't a hardware problem, because everything works with older fglrx drivers (which I'm now using).