As implied by its name, the gallium-msaa branch introduces support for Multi-Sampling Anti-Aliasing (MSAA) for the Gallium3D driver architecture. Specifically this branch makes the structural changes needed to allow MSAA to work within Mesa on the Gallium3D architecture and a context function to set the sample mask for MSAA. However, the Gallium3D hardware drivers themselves haven't yet been hooked-in to actually offer multi-sampling support. Hopefully this will come soon.
While the Gallium3D drivers are faster than classic Mesa drivers (namely the ATI Radeon R300g driver), the ATI Gallium3D driver is not yet faster than the proprietary Catalyst driver. Using MSAA will only drop the frame-rate further, so it will not yet be of any real value to users once the drivers properly are hooked into offering MSAA, but should be useful in the future once a Gallium3D stack supports OpenGL 3.x, have gone through more performance optimizations, and are able to handle more demanding games/applications.
The gallium-msaa branch merge with the underlying Gallium3D changes that went on can be read about on the Mesa development list.