This Phoronix article demonstrates the advances that have been made in the open source ATI/AMD drivers for the R500 and older chipsets. For R600 and newer chipsets, the story is a little different.
Most of the performance improvements in the Phoronix tests came from the Gallium3D driver in the Mesa OpenGL library. When the Debian package for Mesa 7.10 enabled the Gallium3D driver, people with R600 and newer GPUs quickly noticed problems. I saw frequent GPU lock ups but 3D graphics was still functional. Others had more severe problems.
Needless to say, Gallium3D has been disabled in the Mesa package for now. You can only go so far when you are out on the bleeding edge, then you have to back off.