The open-source driver remains a ways from competing with Catalyst especially on the higher image quality settings with Xonotic. Unfortunately, though, because of recent Radeon-specific changes, there's a huge performance regression for R600g when using the high image quality settings with Xonotic on the Mesa 9.1-devel Git code. The performance becomes completely unplayable.
While there is the Xonotic regression outstanding, in a majority of the Linux OpenGL benchmarks carried out, there's very measurable performance improvements to be found between the Linux 3.5 / Mesa 9.0 stack as found in Ubuntu 12.10 compared to the latest Linux 3.7 / Mesa 9.1-devel stack as of this week. These improvements are great to see although for most of the tests, the open-source driver is still having a hard time at running half the speeds of the Catalyst driver. For those soon wanting to play Valve's Source Engine games on Linux with Radeon hardware, your only adequate solution will be using the binary Catalyst driver. The open-source driver is also still struggling with the Radeon HD 7000 series 3D support, power management, UVD video playback, and other functionality offered by the proprietary Catalyst driver.