4 years later and ATI Linux drivers are still garbage
Actually, I have an HD6950 in my box. I had been running the proprietary driver basically because it was set up by default and I was too lazy to change, but yesterday I decided to give it a go. In terms of performance, it is not garbage (I haven't looked at the code). There are actually quite a few advantages to the open source driver.
For one, the 2D operations seem to be significantly faster. I had to screw around with Catalyst on the proprietary driver to get good desktop performance, but the open source driver is considerably snappier out of the box. Also, Gnome Shell was crashing on me frequently with the proprietary driver (usually when doing an expose type event), but this seems to have stopped completely with the open source driver (it has also never crashed with my Intel card on my netbook). Finally video playing seems to have been improved. No matter what I did there would always be some situations where I would get tearing with the proprietary driver, but I never get tearing with the open source driver. There is probably a way to fiddle with Catalyst to get everything working well on the proprietary driver, but I could never seem to find the sweet spot in terms of performance and stability
I'm not a big gamer, but I have a few games. Some games work flawlessly. Some have reduced framerate. One game (the World Forge Ember client did not run at all due to driver problems.
Apart from Ember (which is kind of screwy most of the time anyway), every game I've tried is playable at a reasonable framerate and resolution. I suspect that hard core gamers would not be happy playing some of the more modern windows games under wine, but I don't have any of those to test. On the whole, for a casual open source gamer, the open source driver actually has a better user experience for me. Admittedly, I have a fairly high end card, so I don't know what it's like for a cheaper one, but I don't get a dramatic drop off in performance. The improvement in other areas more than makes up for it in my mind.
Quite possibly for a specific application you have in mind, it's not acceptable, but that's a far cry from "garbage".