Put simply, using Cat 13.11 with my HD 7950 in Ubuntu 13.04 with kernel 3.12-rc6, I can switch to a different tty, but I will be blind since the screen is black and I cannot see what I type. I noticed this when I had GUI lockups when running a wine game, Osu!, in fullscreen which will consistently lock up every time I complete a beatmap. Blindly killing the game will cause a bit of garbage from the game to reliably appear in my tty session and I can go back to tty7 and my screen will finally be un-locked.
I also have issues where I am running something in fullscreen, such as smplayer playing a video, and if I exit from fullscreen all of the Windows are glitched out with garbage. Right clicking, which invokes a context menu, will fix the graphics corruption.
Performance-wise, the horrible performance is expected. Just launch a couple 3D games in Linux and you'll have performance-issue galore in each and every one of them. Any word on when we can expect a Catalyst 13.11v2 Catalyst 13.12?