Having seen Avatar in 3D last week, I suppose comment is almost obligatory. There has been a wide range of reaction to the film I have seen, ranging from ecstatic to downright derogartory.
I liked Avatar. The plot is both recycled (mainly from Dances With Wolves), and paper thin, and signposted in big flashing neon, with characters painted from the broadest sterotypical brushes, but if you are going to get hung up on those things with this movie, you are both coming at it from the wrong angle and over-reading it. This is meant to be big concept sci-fi escapist entertainment, not an insightful commentary on the effects of colonialism and exploitation on indigenous societies. While those aspects are present rightly or wrongly, they aren't the raison d'etre, and focussing on them I think is taking things too seriously.
The plot exists as a vehicle for the spectacle, and as far as spectacle goes, Avatar is seriously epic. While it isn't entirely made of awesome, it is never actually bad, and can rightly be regarded as a standard setter in a technical and production sense.
The virtual realisation of Pandora is so good it resulted in total immersion and suspension of disbelief for me, especially in 3D. A lot of the time I forgot that the environment and characters on screen was totally CG, and this is the film's greatest achievement. At times this is spectacular, and never less than impressive.
More importantly, the CG allowed the Na'vi characters to be realised much more effectively and seamlessly. Its a simple concept but I enjoyed immensely the fact of the Na'vi being 9 feet tall (only fully apparent when appearing alongside humans), since it emphasised their otherworldliness subtly without being dramatic.
There is a lot of detail and subtlety on screen (in fact probably too much, the film could probably easily be an hour shorter, and has pacing issues at times). I liked a lot of the smaller details (even when they were wrong, like Dr Augustine's terrible pipette technique in the background of one of Sully's diary pieces to camera), some simply for the fact they were there (like removing the engine covers from the helicopter during the escape scene. It might have only been for dramatic reasons, but it highlighted nicely that you very seldom jump into aircraft and make them go, especially if they have been bedded down for the night).
Like others I think Sully's assimilation into Na'vi society was a little too quick. A year rather than 3 months could have been allotted to this without the film suffering from it. I don't have an issue with his crucial plot point insights though like other reviewers have. His way of thinking compared to the Na'vi will by definition be alien. Its not impossible for him to come up with things that may never have occured to his hosts (and lets face it, how often have you had another pair of eyes bring a solution to a problem that was obvious in hindsight).
Its a long way from perfect, but for all its flaws, Avatar works, and works spectacularly well at times. Worth seeing for the imagery alone.