Phoronix pits Intel's open-source driver against Apple's Intel graphics driver for Mac OS X, you can check out the results over here.
Thanks to recent advancements by Intel's Open-Source
Technology Center, the open-source Linux graphics driver not only
supports more OpenGL 3.0 functionality than Apple's Intel graphics
driver for Mac OS X, but the performance is more competitive. In some
cases, the OpenGL performance is now superior under Linux with the
open-source driver that is developed by Intel in conjunction with the
free software community. This article is looking at the performance of
Intel Sandy Bridge graphics under Mac OS X 10.7 "Lion" and Ubuntu Linux.