This is a remarkable technical achievement - motion-capture driven CG characters that are natural and expressive, in a richly-imagined forest and jungle environment. I found sitting in the front row center for the 3D version (we got to the theater late) a dizzying experience, but did better hiding off in a corner.
The story is completely predictable, the characters unmemorable except for the militaristic villain, who is thoroughly detestable. The situation deserves a much more complex and involved back story than it is given. See this film for the beauty, excitement, and action of the visuals, and don't bother to read the subtitles -- the dialog in the aliens' language is no better than that in English.