I have been working with WPMUDEV for almost three years now. It started with using some of their premium plugins and now I've moved all my hosting to them (25+ sites and growing). Their support is outstanding. There have been, maybe, three or four times when a problem wasn't solved on the first go-around. The team is definitely growing (I used to get pretty much the same group of techs whenever I connected), but everyone is knowledgeable and helpful. I'm a huuuuge fan!