We finally saw John Carter on Saturday night. It was great fun! To me, it was reminiscent of the Saturday afternoon matinee, "Family Classics" style of film, like Journey to the Center of the Earth or Jason and the Argonauts.
It strayed from the storyline of A Princess of Mars, but not terribly so. If you enjoy sci-fi/fantasy adventure movies, I think you'll like this. (we saw the 2D version)