GGX is a microfacet shader for specular components that should solve some of the problems of the current microfacet model used for glossy specular, which produces too much of a 'plastic' look when used for reproducing metallic materials. Also another issue of the current model is that when maximum exponent is used for sharp glossy reflections, it does not converge to a true mirror reflection like in other software and some glossiness always remain. If implemented, this could solve also this feature request, as a glossy shader with maximun exponent is the only way to get sun disk on mirror like reflections from sun lamps and sun components in sunsky models in YafaRay.
I am going to do more request about texturing in the coming weeks so this is only one example of improvements we could do in that front. Greetings.