Just watched John Carter, on Spike TV, and I found it was a very good movie. I don't understand why it was such giant failure at the box office at the time of its release in theater. I liked it very much, and the story telling structure is very clever, in my opinion. Of course it requires to be watched until the very last second to take all its dimension, but I don't want to spoil.
Now, I never read the book, and may be the film is disappointing compared to the work of Edgar Rice Burroughs. However, I believe that on its own, it's good movie, and fulfills expectations greatly.
So, my recommendation, if you hadn't watch it yet, then give it a try, and stay up to the end.