I enjoyed it immensely. It's relatively well-written, visually spectacular (I weep for those just watching it now who didn't get to see it in 3D in the theatre - it remains the only movie I've seen that really used the effect with subtlety), and the world-building aspect is really interesting. But I always enjoy media where a central theme is learning about an alien culture, so I'm probably biased in that.
The main criticisms I've seen of it were about how it was Pocahontas in space. Which it totally is. How this is necessarily a bad thing, I do not know.
That and criticisms about the notion that the native culture can only be saved by an outsider. But I've always found that criticism especially silly. For one, an outsider attempting to integrate with a society will necessarily see some ordinary parts of the culture more easily, since, to him, they will be exceptional. So it is not at all unthinkable that an integrated outsider might, in some respects, be in a particularly good position to deal with these problems. Add to that the fact that the humans had been studying them for some time, while they had spent virtually no time studying the humans, and you have very strong justification for needing human help to come out victorious. So no, I don't think this is a strong case of the brave white man saving the imperilled natives who are inherently helpless to save themselves. I think the film presents plenty of justification for having an outsider save them. If anything, the film is about the necessity to attempt to understand cultures and about how this can be mutually beneficial - you get to understand other cultures, and you understand your own by letting them look at yours. I think people seeing these as problems tend too often to treat blatant xenophobia as the only solution to cultural imperialism.