While the Nouveau Linux driver may be a reverse-engineered incarnation of the NVIDIA binary Linux driver and is developed without any support from NVIDIA Corp, the Nouveau experience wasn't that bad for Doom 3. Using the Nouveau Gallium3D driver and Linux 3.2 kernel across several generations of NVIDIA GeForce graphics processors was a decent experience. There was never a GPU hung or any rendering issues like there was with the Intel Sandy Bridge graphics, which may mean those issues are bound only to the Intel driver or the classic Mesa code-paths. Fortunately, Intel is the only major vendor still backing a classic Mesa DRI driver rather than Gallium3D.
The tested graphics cards included the GeForce 8400GS, 8500GT, 8600GT, 9800GT, GT 220, and GT 460. This GeForce selection spans several generations with low and high-end components. Upon the release of the open-source Doom 3, I can easily expand the NVIDIA comparison if there is sufficient interest. The Nouveau Gallium3D stack was able to provide a playable experience at 800 x 600 for the NVIDIA GPUs except for the low-end GeForce 8400GS and.... Fermi. It seems the Fermi "NVC0" Gallium3D driver is in bad shape as the GeForce GTX 460 should be the fastest performer among the graphics cards tested, but here it was as bad as the GeForce 8400GS. However, as I have said many times, using the Nouveau driver is like Russian Roulette in terms of reliability and usefulness.