Hi!
I believe that building a stretchy facial rig is cool but it's not enough to make the animation to look organic and natural. The audience must feel there is a structure underneath the CG skin. Plus keeping the style of the character is crucial. Not just letting the joints push or pull the vertices where they like and how they like. The CG elements have linear transformations while the natural elements always move in arcs. The artist must teach the vertices of a CG face how to behave like an organic face!