My friend and I went to see this Monday night. We both came away feeling like this movie was extremely well done. I had never read the original books, but thanks to public domain, all the Edgar Rice Burroughs books can be downloaded on iBooks for free. Last night, I devoured "A Princess of Mars", the book credited in the movie.
Needless to say, I'm glad I saw the movie first. "John Carter" is another Reboot movie where massive changes were made to the story. Considering the originals are now close to one hundred years old, some refreshing is expected, but wow.
If you read the John Carter stories as a kid and think that all Science Fiction owes a debt to them, then I suggest you don't pay money to see this in the theater unless you can live with everything you know is wrong.
One thing that irked me after seeing the movie was the way the moons of Barsoom were imaged. Deimos should barely be visible while Phobos would move across the sky at quite a clip. Burroughs was aware of this and wrote it into his stories, but the movie treats them like the Eiffel Tower: if it's night, we must see two moons side by side so we know we're on Mars.