James Cameron has always been an interesting film maker . Although on one level most of his films could be classified as pulpy genre-related fare, they usually have a more interesting subtext lurking below the explosions. Previous Cameron films have investigated such themes as mother/daughter relationships, humanity's fear of the unknown, musings on fate and predestination, and whether it is morally acceptable (and perhaps even admirable) to slum it with a good looking lower class boy for a few weeks before you get married even though an ocean liner might not be the best place to do so. So it is with a heavy heart that I have to say Avatar is a slight disappointment.
The 
planet of Pandora (Who names these planets? What were they thinking?) has some stuff that humans want to mine. Unfortunately, the best place to get it is right on top of where the indigenous population (8 foot tall skinny blue people called the Navi) live in harmony with their world. The Navi are distrustful of the humans, so in order to investigate the Navi a bit more, the humans create the titular avatars - mindless Navi bodies that certain individuals can "drive around" remotely. The main character is just such an individual, and he (or his avatar) quickly becomes involved in the local tribe. Although the humans would prefer that the Navi move on without violence, it is clear that a military solution, led by a crazed marine, might be more expedient...
It is almost impossible to spoil anything about Avatar's plot, no doubt you have already guessed the direction it which it unfolds. It is a shame that for all the risks involved in making what is apparently the most expensive movie ever made (it certainly looks like it), the story is as safe as an after-school special. The film could have made some interesting points about colonialism, or environmentalism, the military, or even feminism, but instead chooses to unspool a conventional yarn where the good guys are selfless and the bad guys are crazy and evil. It is not that is it a bad story per say, just something we have all seen many times before.
I saw Avatar in 3D, it is by far the best 3D experience so far. The lush jungles and mist-shrouded peaks of Pandora look amazing - Avatar is simply the greatest visual treat I have ever seen. The contrast between the sharp grey lines of the human base with the colourful, glowing environment outside is very well rendered. James Cameron has always been interested in portraying technology and Avatar is no exception - a nice touch is that all of the displays that the humans use during the movie are also in 3D. There are a thousand little details like that I loved about Avatar, it is just a shame that the whole thing isn't as great as the sum of its parts. However, anyone who shares Cameron's love for helicopters and giant robots and things being blown up by helicopters and giant robots will be thrilled.
Highly recommended if you can see it on the big screen in 3D. Otherwise only recommended if you like this sort of thing (but who doesn't?)