MINOR SPOILERS
At a minimum, you have to say Cameron gets an E for effort. This is a very bold and original movie, especially technologically, but also in concept. No SF movie since Stars Wars can match it in that respect. But was the 3-D worth the trouble? I’m not sure. While there were some scenes where it added some depth and realism, there were others where it was more like annoying. As seems to be customary these days, the action scenes were mostly photographed in the blurry sort of fast motion that made the battle scenes in The Lord of the Rings such a disappointment.
The failings of the movie derive from its attempt to be too many things at once: adventure story, morality play, and action movie. My favorite part (roughly 90 minutes worth) was the adventure story of how the hero gets sucked into the world of his avatar.
The morality play features a conventionally greedy corporate exploiter, a couple of boring villains, and some inexplicable converts to the eco cause.
The action finale seems silly, boring, and annoying to me. I was really hoping for something a bit clever. Nope.