While the latest Mesa code "works" with the Unigine Engine, in terms of supporting the necessary OpenGL extensions for basic support, its performance is still a long way off from being usable and there isn't support for all of the features provided by the binary AMD and NVIDIA drivers.
When running the demanding Unigine Tropics tech demo on the Radeon HD 5750, the GNOME Shell performance plunged while the other configurations performed at the same pace. With the NVIDIA driver, the performance under KDE came out slightly ahead, but it was not a big difference.
GNOME Shell 3.0 is rather a mess right now in terms of bugs on various open and closed-source graphics drivers and in terms of causing performance regressions. The Unity performance is not too surprising because Ubuntu is just relying upon the mature Compiz as its compositing window manager, but there is the recent regression causing the open-source drivers to break hard. One other interesting takeaway is how the proprietary NVIDIA driver tended to perform the best under KDE with KWin, but we'll see how that goes in relation to the other drivers once KDE 4.7 is released and the KWin changes it brings, including suspend/blocking compositing.