Hot off the presses!
It's still not usable. At least, not on my mid-2011 iMac 27″ Quad Core 3.4 GHz i7 with 16 Gigs of RAM, a 240 Gig SSD, among other things.
When I play video, after 5, 10, or longer the video slows down and I lose audio altogether. This seems to happen more frequently in fullscreen mode.
I thought maybe if I downloaded the 10.8.1 full version that it might solve the problem. It did not.
Back to the safety of Lion (10.7.4) for now.
Update (9/25/12) – Mountain Lion Update 10.8.2 has been released. It still does not address the serious audio/video problem. According to folks in the Apple forums, Apple seems to be ignoring the issue. My guess is that they know about it and may not know how to fix it. Since this problem is among certain Mac models I would guess that its a driver issue. Perhaps, if we start replacing audio/video drivers with those from Lion it would fix the problem? Oh well, crossing fingers again.