Well, I would say that, except perhaps "Riot At Xavier's"(on which he does kick ass, probably because he's mainly focusing on characters he designed) it could easily be argued his X-Men work is the least of his work. But I can't feel too sad about that. I don't think his heart was as in it as, say, ALL-STAR SUPERMAN or FLEX MENTALLO, or any number of other really good work he's done.
But really, it's X-Men. How far could he really go with it? Morrison tried to go pretty far and then it was all erased.