One of the new Cycles features in 2.78 is Multiscatter GGX.
When should I use it instead of the default? (Bonus question: and when sould I use the other possibilities in the Glossy, Glass and Anisotropic BSDFs?)
So far I found only the following
Essentially, the improvement is that unlike classical GGX, which only models single scattering and sets the contribution of multiple bounces to zero, this new model performs a random walk on the microsurface until the ray leaves it again, which ensures perfect energy conservation. In practice, this means that the "darkening problem" - GGX materials becoming darker with increasing roughness - is solved in a physically correct and efficient way.
The downside of this model is that it has no (known) analytic expression for evalation. However, it can be evaluated stochastically, and although the correct PDF isn't known either, the properties of MIS and the balance heuristic guarantee an unbiased result at the cost of slightly higher noise.
This sounds too technical to me. I understand the not-darkening part, but that's all...