Saw John Carter.
Nothing stuck out as grating or terribly obnoxious (there's even a little winking joke with the audience about belaboured melodramatic speeches in Dejah Thoris' first appearance), which is actually more than I can say for most modern films like this!
As some people mentioned, Kitsch's delivery was a little flat, but when I say "a little" I mean that literally: it's a terribly minor complaint and doesn't really detract from the film. And sometimes it actually kind of just works.
I barely felt any of the mid-movie drag some people mentioned. The whole movie went by quite quickly for me. You can see where some elements from later books have been patched in (the stuff with the Tharns and the 9th Ray), but it's not too jarring.
That mostly sounds like a list of "Complaints I could have had, but didn't", so I will also add that in addition to an overall sense of fun the fantasy/sci-fi elements were played well without being overdone. I can echo that the effects and visuals were very good without being overdone. The characters are all likeable and serviceable in their own ways. Nobody stands out (except maybe Willem DaFoe as Tars Tarkus), but they all do a sound, serviceable job. And that's actually pretty respectable.
All in all the movie is actually quite decent and a lot of fun!
The original books were on my to-read list since before I heard of the movie, but I think I ought to move them up.