Open-source Intel Linux developers really could care less about the i915~i945 generations, especially as they aren't even committing new significant features to Ironlake. However, Google and others have been maintaining this community-based "i915g" driver. The main benefit of this i915 Gallium3D driver over the current official i915 "classic" driver is that due to leveraging LLVM it has faster performance in some scenarios, particularly with this older Intel hardware not having vertex shaders thus needing to do it in software.
Google has advanced this driver a lot and shipped it in their Chromebooks shipping with older i945 graphics. Its performance is decent considering the hardware.
With not much activity going into Intel's i915 driver and the i915g driver providing new features over the original driver, the Gallium3D driver in this case is really the better choice. Developers even brought sRGB textures so the driver could expose OpenGL 2.1 support.
On the Mesa mailing list, David Airlie expressed, "the thing is distros should probably be using i915g at this point over i915..."
Daniel Vetter additionally commented that a small change would be needed for handling old i830/Gen2 support to link to the older Intel classic driver over the Gallium3D driver where this i8xx hardware is unsupported. Chris Wilson this morning now committed a change to the xf86-video-intel driver for changing around this i830/i915 DRI preference so that the Intel "Gen2" graphics will still work with 3D when this Mesa change ultimately happens.
New benchmarks of the i915 Gallium3D driver will likely come soon on Phoronix, but until then, check out Ivy Bridge benchmarks of the new Intel Gallium3D driver.