All things considered, it's not too bad. I very much appreciate that the film retains the book's framing device of John Carter being Edgar Rice Burroughs's "Uncle Jack," who left instructions to him regarding the publication of his recollections of his exploits on Mars. I also like that the film doesn't shy away from Carter's past as a Confederate soldier who went West seeking a prosperous new life for himself in the aftermath of the Civil War. I'm not as keen on the additions made to text, in particular the implication of a tragic backstory. Mind you, that's a longstanding pet peeve of mine. For some reason, Hollywood seems to feel the need to invent past tragedies in order to explain a protagonist's current life. It's notably unnecessary in the case of John Carter, who, in the novel, remembers little of his earlier life.
I'm still planning to see the film, of course, because, for all its likely deviations from the source material, it still looks to be more faithful to both the letter and the spirit of the Barsoom novels than any of the Conan films. Once I've had a chance to view it, I'll post my thoughts here.