If I could sum up this film in one sentence, it would be "I kept waiting for the story to start, and then the movie was over."
Let's get the obvious stuff out of the way. Visually, the film is stunning. From the very first scene, where we see Martian airships battling one another, you really get the impression that you're seeing a completely unique culture and technology, and the creatures (especially the "green martian" Tharks) are exceedingly well done. My only complaint in the visual department is that Deja Thoris's eyes are sometimes a brilliant, cobalt blue, and sometimes they're the regular brown of actress Lynn Colins. Obviously, like David Lynch's Dune, they forgot to digitally color her eyes in some scenes.
I can't fault the acting, either. Given the type of movie this is, you don't go into it expecting Casablanca, but all of the actors and voice-actors do a credible and convincing enough job. Ciarán Hinds, who played Julius Caesar in the HBO series Rome a few years back, stands out as having given an exceptional performance, and brings a terrific gravitas to the screen whenever he's on it.
If anything, my biggest issue with the film is the story. The screenplay has no less than three credits, and I think it shows. We see a lot of setup, and then, just as you expect to see the plot take off, it slows down and we see yet more setup. Then just as we're about to see the story take off, down it goes and we're given more set up. By the time the finale arrives, I at least was left wondering just how we got there.
For example, John Carter spends the first half of the movie trying to get back to Earth, and then suddenly decides he's going to help Deja Thoris. Why? What caused his sudden change of heart? It seems to be a mystery, just as is the sudden change of heart of the Thorks to intervene in the Red Martian civil war.
I must say I also found the music completely forgettable. The simplistic color-coding of the good city and bad city was helpful, but some sort of memorable theme song for one or the other might have been a nice thing, too.
On the whole, I didn't hate this film, but neither was I blown away by it. Considering I spent $15 on a single ticket (IMAX 3D bumps up the price just a tad, but the 3D didn't seem to add anything to the experience), I was a bit disappointed by what I got. I'd give this film two and a half stars out of 5.