In the GTK+ performance measured by GtkPerf, the open-source Radeon driver was faster at rendering in most cases. Granted, we hadn't compared the CPU load during this, but the open-source driver overall was about 10 seconds faster than the fglrx driver in the total time needed to complete 1,000 GtkPerf trials. In the individual GtkPerf tests, the results varied but were within a few seconds of each other.
While we hadn't thrown any of the old fglrx drivers into today's tests, it looks like we're approaching a state where the R300/400 GPUs powered by open-source software has a comparable frame-rate to the ATI/AMD binary blobs based upon their old code-base. There's still much work left to do, seeing as a Radeon X800 series part unless using the fglrx driver cannot even properly handle Doom 3 let alone Enemy Territory: Quake Wars. Though once AMD gets around to releasing the additional GPU specifications that they have promised (as right now they have only released about 900 pages for the R500/600 series), we will hopefully see a number of improvements. Neither side has a perfect solution, but with the 8.42.3 driver still running fresh off the new driver code-base and the just-implemented AIGLX support, check out the Phoronix Forums about a number of bugs that people have been experiencing with this Linux driver.