During a session this morning at XDC2012, Ian Romanick of Intel OTC made some brief comments about the Gallium3D architecture. Lucas Stach was presenting on coming up with hardware-independent DDX drivers via GLAMOR or using Gallium3D state trackers like Xorg and XA. Intel's Linux graphics driver supports GLAMOR, but its classic Mesa driver prevents the XA/Xorg state trackers from obviously being used.
Ian Romanick, one of the well-known lead OpenGL developers as part of Intel's Linux graphics team and is also involved extensively with the Khronos Group / OpenGL ARB, bashed on Gallium3D during comments.
Intel has avoided Gallium3D and it looks like they will continue to do so for the foreseeable future. While some users are fond of Gallium3D for its "universal" architecture and state trackers coming about for doing more than just 3D/OpenGL (such as the aforementioned 2D, video acceleration, GPGPU/OpenCL, D3D, etc) leads to way too much overhead. Other users also think that Gallium3D is faster or want an Intel Gallium3D driver simply because it's the latest "popular" topic as of late.
Romanick believes the reason why the open-source Gallium3D graphics drivers are so slow is attributed to way too much CPU overhead with Gallium3D attempting to do/support too much.
Meanwhile Intel continues employing many developers that are heavily invested in their existing Mesa DRI driver that has advanced greatly over the years in terms of performance and features. For those interested in open-source Linux graphics support, the Intel driver is basically the best and most well supported.
More XDC2012 videos are forthcoming.