Dropping in with some information that hopefully can be somewhat useful, even if outdated.
I haven't yet tested the new Mesa 12, but I was unable to play games on Steam (at the time, Borderlands 2, it would hang after entering the game and zooming with any weapon) and I had to disable GPU acceleration under Chrome. Kodi also sometimes had some issues, but not that often.
At the time Steam at least didn't hang my machine, but with GPU acceleration enabled on Chrome I got a hard hangup on the whole system every 2 days or so - ever since I disabled that my system has not required a single hard reboot (although Chrome is unable to render some stuff correctly).
I will keep tracking this and when I get to update my system I will look for an easy reproducer to this - pointers for known good tests are welcome. ;-)