I know 2.71 hasn’t even officially been released yet, but in 2.72, there’ll be a new distribution model for glossy shaders, Ashikhmin-Shirley, and all three models have now been made available for the anisotropy shader too. I played around a bit and came up with this little comparison:

(Best to open each of these in a new tab to compare them easier)

Anisotropic Shader

Ashikhmin-Shirley:

Beckmann:

GGX:

Glossy Shader

Ashikhmin-Shirley:

Beckmann:

GGX:

Ignorant Observations

As far as I can see, it’s a sort of middle-ground between GGX and Beckmann anisotropy – it spreads the darker values much better than Beckmann, but doesn’t have tails as long as GGX. As for the glossy, it seems very similar to Beckmann in the low-ish roughness values, but with very high roughness it’s a lot more like a diffuse surface (which I suppose is technically more accurate).

I actually don’t know anything about this new model, so if you can educate us all, I’d be keen to hear it.

For some more info, see the task on developer.blender.org