I'm hoping to write two reviews of this movie. Here is the first, which I will keep spoiler-free. The other one will have all spoilers behind a cut.
First off, the movie is nowhere near as bad as the reviews claim. It also isn't the awesome blockbuster it was apparently hyped to be either. The Last Airbender is not the next LotR, but it's still way better than Twilight.
The CG was well done, and most of the effects made sense in the context of the world. Some of the costumes were very good, while others I was less fond of.
There were two major weaknesses of the film: direction and screenwriting. M. Night Shyamalan is just not a good director. He seemed to leave the actors to their own devices most of the time, and it just doesn't work with a cast that is relatively unknown or inexperienced. Also, I hereby revoke his permit to use the slow-motion camera. The movie was too short as it was, no need to make it shorter by stretching out shots unnecessarily. Fight scenes went back and forth between awsome and kind of silly, which is sad because fight choreography was one of the strengths of the animated series.
The screenplay had a lot of problems. Condensing ~10 hours of TV into a feature film is hard. Without getting into detail, I will say that my biggest issue was that it repeatedly broke the rule, "show, don't tell.". However,
j_b said he could follow the story okay without having seen the series.
Overall, I'd give it a B-. Totally rentable, but fine if you're looking for an excuse to go to the theater.
Oh, and the best actor in the film is Aasif Mandvi, who you might recognize from The Daily Show. This comedian can also do drama, and he made the best of some poorly-written, repetative dialog.