I only think I've ever gotten an encounter twice in the hidden grottoes, and it was the same Pokemon both times. If I'd spent more time with my Black 2, I reckon I might've encountered more, but I generally never had any luck with finding anything in them. I actually searched through them quite often when searching for specific Pokemon, so it's not like I didn't try :p
I think it's a neat concept, but I think the encounter rate should've been increased a bit. Or I might just've been extremely unlucky. In any case, encountering my first Pokemon in one of them was actually kind of a surprise, since I had no idea what they were at first, and OW sprites in the past had mainly been reserved to legendaries or rare Pokemon. So yeah!