Bottom (top) line: You have to see this movie. In theaters. Even if you might not like it. Because no other screen will do it justice, and you can't catch it later.
This movie is pretty much like everyone else has describe: amazing visuals, overdone plot. If you've seen Pocahontas, you know what's going to happen in Avatar.
But that doesn't matter. The reason you see this movie isn't because of the plot. While watching, you don't even care that it's unoriginal. What truly matters are the visuals. I'd disagree with one of the posters below who suggested that the 3D was not necessary. It is definitely subtle, but that's what's amazing. Everything looks more real, but you don't even realize why. That's artistry.