Ok - first the "not so good". The story is old: big, bad corporation trying to take land from poor indigenous species is aided by former bad guy now turned good by the lure of the pure lifestyle of the "people".
With that said, the "good" was that the story was well-told with sufficient acting chops thanks to Sigourney Weaver and Giovanni Ribisi (with honorable mention to Michelle Rodriguez).
The "best" was the cinematography. I saw it in IMAX 3D and can't imagine seeing it any other way - the art of the imagined world, the creatures, the attention to...