Nexuiz is one of the few open-source games that offers impressive graphics, but this graphics intensity really takes its toll on Mesa. Running at 2560 x 1600 with the X1950PRO, the average FPS with the Catalyst driver was 82 but just 22 if using the current open-source code. A frame-rate of 22 FPS really isn't playable. The X1950PRO is one of the fastest R500 graphics cards, so anyone using an R300/400/500 graphics card can stop thinking about playing Nexuiz at 2560 x 1600 until the open-source driver improves. Those not using a Samsung SyncMaster 305T or another very high-resolution display, playing at a lower resolution should offer better performance.
Turning to the 2D performance, the open-source stack strikes a win (albeit by a small margin). Using EXA acceleration with the xf86-video-ati driver, the GtkComboBox performance was slightly faster than the fglrx driver.