CAUTION POINT ALPHA: Do not watch this movie if you are not watching it in 3D.
I'm serious, don't even bother: go the right way or don't go at all. All this movie's impact is in the special effects, the 3D, and the action. If you're not wearing your 3D shades you've lost much of what's special about it. I actually flinched to dodge a tear gas grenade at one point. The 3D stuff isn't driven by shock "made you jump" stuff, though: the tricks are constantly being used to subtler effect. I will say, however, that this beats the hell out of your eyes. I had to take my glasses off from the strain at a couple different points during the overly long movie: it's like Virtual Boy overexposure, but with more colors.
I expected the CG people to be all the way in the Uncanny Valley, but unlike, say, the recent Christmas Carol remake, these CG people are actually convincing. Of course, the movie makes sure to almost never have live actors and CG ones in the same scene-- save a few shots towards the end-- but the CG, standing on its own, is now strong enough that these sections come off exactly as "real" as the scenes with live actors do. You have to hand it to Cameron: the bar is officially raised.
CAUTION POINT BETA: You have seen this entire movie before.
The whole thing, beginning to end. Man from dominant civilization infiltrates the Other, learns their ways, becomes accepted, sleeps with a girl, changes sides on the spot. It's done well enough, but like everybody has been saying, you're in this for the visuals. You will be stunned by an unparalleled visual assault, but not surprised and certainly not moved.
CAUTION POINT GAMMA: A number of the plot points of this movie are stupid.
A couple of friends said "sure, go ahead, spoil us" over dinner at Go Go Curry, and by the end of it they were laughing their asses off. There are a number of seriously groan-worthy "I can't believe you expect me to go with this" moments in this movie, most of which are packed together at the end.
Characters aren't much better: there's the noble, peace-loving, nature-respecting and utterly perfect natives (the Na'vi), the puppy-stabbingly evil humans (all but the hero and his friends, the nerds in a world of uniformly evil marine jocks), and they're all completely hollow. It's hard to sympathize with such caricatures.
The final scene also ensures that this will become the official furry recruitment film for the next fifteen years.