I still say that Disney's marketing team screwed the pooch on this one by dropping "of Mars" from the title, leaving the uninformed viewer really confused as to what in blazes the movie is even about.
I just saw it the other night, and found it thoroughly entertaining and well executed, but with a plot that was pretty rough around the edges. In the end, I enjoyed it more than I expected, though that may be aided by the fact that there's been a dearth of adventurous science-fiction/fantasy in my viewing as of late...
This quote from io9's review sums it up pretty well for me:
The real question is should you watch the movie to catch those moments of greatness, or skip it because you know you'll be frustrated by its problems? I would err on the side of watching. This is an imaginative bit of fun, and it looks fantastic splashed across a giant screen. (Do not, however, make the mistake of seeing it in 3D. It wasn't filmed in 3D, and the 3D frankly looks like crap, spoiling the visuals at every turn.) John Carter may not be the gamechanging retro-futuristic epic we were hoping for, but it's still worth checking out.Though I disagree with the author on one point: I rather liked the frame story!
On another note, the original Edgar Rice Burroughs story, A Princess of Mars, has lapsed outside of copyright in the US, and is available on Project Gutenberg! I'm partway through now, and it's intriguingly different enough from the film to be worth a read if you're interested. Very much a product of the time it was written, and if you're fascinated by the history of the genre, it's cool for that reason alone.