I have a small one and a bigger one, I like them a lot. I haven't used them much, just when Boomer was seizing on the way to the vet as a precaution, and to do my boss's chow's nails (she did hit my hand with the muzzle and it was fine). The small one fits Fiona fine, but I have doubts that it would stay on if she really tried (she doesn't need it either, I just got that one for free from work).
I do believe that conditioning the dog to the muzzle with good and yummy things (clicker work) is better than sticking it on them and 'forcing' them to accept it, for what it's worth.