They suck for sure. Nothing much required to prove than showing up the status of xorg/gallium drivers for ati radeon as of 9th May 2012. Have a look at the stuff (marked red) that's not working perfectly. The current situation in modern ati graphics chips almost wards off anybody from using linux. And it doesn't seem to go anywhere till couple of years. Pity!
* "DONE" means that it is implemented and relatively bug-free.
* "MOSTLY" means that it is implemented but has some known bugs.
rest here