John Carter is based on Edgar Rice Burroughs' (of Tarzan fame) 1917 novel, A Princess of Mars. I haven't read the book. However, keeping the age of the story in mind, this movie is highly entertaining, if very Hollywood-sy. The story is a nice blend of sci-fi and fantasy with a decent plot and a nice twist at the end. The action is mostly good, the humour mostly funny and the clichés mostly bearable. But the acting could have been better.
The best part of the movie is the Martian race of Tharks who were well imagined, designed and animated. I enjoyed this movie thoroughly and wish that a few sequels crop up soon.
Recommended viewing especially if you are into sci-fi / fantasy. I should probably rate this lower, but I've tacked on a star purely for its Gemütlichkeit-ness. Now I'm off to read the book(s).