I took the kids a couple days ago. To be honest, I don't get the hostility to it. It might be faint praise, but it's significantly better than The Golden Compass or Eragon, if only because it is pretty short (94 minutes running time).
I think part of the negative reception might be because of the 3D version. We saw it in a normal 2D and thoroughly enjoyed ourselves. I would give it a B-. The acting is not strong, but Shyamalan knows the kids' limitations and does not ask them to do too much. The plot is limited somewhat to introducing the story and the Airbender world, but there are a couple nice character lines (Aang, the title character; and Prince Zuko, one of the antagonists) that play out well enough. In any event, I was hooked into the story.
Go see it--in 2D--it needs to make enough money so that they make the sequels.