I hated Supersize Me for the same reason I've grown to hate Michael Moore films. The movie starts out and Morgan's already decided that McDonald's is evil... the rest of the movie exists just to prove his point. His girlfriend, from the outset, is pushing her vegan chef agenda and Morgan acts overly disgusted when he eats a Big Mac as if it's vomit on a bun... there's absolutely no attempt at objectivity in any part of the movie.
Same with Michael Moore. Oh, the war in Iraq is evil and here, let me show you what it was like before the war with some scenes of people sipping coffee on a sidewalk in Baghdad while a kid flys a kite in the background ... see how great Iraq was before we stormed in a blew everything up?
I think it's unfortunate because I think both guys are intelligent and are taking on important topics with the films they make. It's just too bad that their egos have to be a character in their movies because, more often than not, it spoils the whole experience for me, even when I agree with them.