I wear one, it would be possibly be hard to get a good airway if you were in an Avy trying to get it in position. I wear mine with mouthpiece under helmet ready to bite. If in terrain with a chance of sliding, I go ahead and get mouthpiece in place and basically breathe "around" it until needed. So far I haven't had an opportunity to test it out, and I'd really be fine with it staying that way.
I really like the theory behind it, thought it was a good investment, still do. My only regret was not getting it incorporated into pack.