I've always been a fan of Cameron's work and Avatar is right up there with Aliens and True Lies for me. The story is very simple, the characters very familiar archetypes, the plot is fairly predictable, but the storytelling is wonderful. We're drawn into his world and taken on a very satisfying emotional journey.
I don't want to give too much away but the key phrases I've heard around the net describing this film ring true to me: Pocahontas meets Dances with Wolves meets the Smurfs... I'd also add in Last of the Mohicans and Starship Troopers. Like with the very best science fiction Avatar relates to both our past and present, dealing with important issues, without beating us over the head with them.
My only word of warning is to not bother with the 3D version. I saw it today and quite frankly gained nothing extra from the experience except for a pair of glasses and a slight sense of disappointment. I'd love to hear from others about what they thought of Avatar 3D, but I was underwhelmed. Avatar was a fantastic film, despite the 3D, not because of it. Maybe it's just me, but 3D did nothing to enhance the film apart from occasionally thrusting a bit of foliage in my direction. Though the novelty of subtitles vaguely hovering in front of the screen did amuse me.
For those worried about language issues, there are no English subtitles for some of the alien dialogue, but you won't miss too much. I was able to follow most of the Korean subtitles for those parts, but then most of it is then spoken in English afterwards (or the meaning is made clear by facial expression and tone).
Let me know what you think about Avatar, I'd love to hear your reaction to this Science Fiction CGI Spectacular...