Phoronix compared the performance of the open-source Mesa 3D stack and Catalyst Linux Suite with the fglrx driver on the ATI R500

"With Mesa 7.1 having been released this week and the open-source R600/770 3D support just around the corner, we've taken this opportunity to see how the open-source Mesa 3D stack compares to AMD's monthly-refined Catalyst Linux Suite with the fglrx driver performs for the Radeon X1000 (R500) series. In this article are Mesa 7.1 and Catalyst 8.8 benchmarks for the Radeon X1300PRO and X1800XL graphics cards."

>> ATI R500: Mesa vs. Catalyst Benchmarking