OK, first things first: if you're a book purist, stay away from this movie. Then again, you probably thought the Peter Jackson adaptation of The Lord of the Rings was crud, too.
If you're OK with pragmatic adaptations, it's pretty good. Definitely a better adaptation job than, say, Starship Troopers - which was fun as a movie, I admit, just a lousy adaptation.
Most of the plot is from "A Princess of Mars", with the sixteen tons of exposition cut for expediency (aka pacing). It also throws in elements from later books and reshapes other things, probably because this kind of movie is always a gamble. It can and likely will do fantastically in the foreign box office, but if it doesn't gross "enough" domestically (whatever "enough" means to the suits, anyway), there will be no sequel. (Yes, the ending hints at one, but we know how that works...)
As a movie in its own right? It's fun!
The pacing could be a wee bit better in the beginning, but that's the nature of the beast. This story really does need that kind of buildup to pull you in. As for the tale itself, there's a good range of emotions going on, with some especially funny moments.
The production designs: Wow. If you're into Steampunk like Yours Truly, the Victorian-era setting of the opening and closing will have your mouth watering, and once we get to Barsoom... hoooo boy. The alien life forms are wonderfully done, too.
I've heard people complaining about it being too much like Avatar or Star Wars. Guess what, people, ERB created most of those elements you now expect in your science fiction back in 1912. This is a story that's spent a century in Development Hell, remember?
I would say go see it, expecting only to have a good time. There are worse things to spend an afternoon on (weekday matinees FTW!). Twilight* comes to mind.
*I hate it so much I need to go brush my teeth after mentioning it...