I'm probably not adding anything that hasn't already been said but I was totally blown away (in a good way, not in a "Hurt Locker" way). It's a good story, with a lot of subtext (both environemental - think "Sierra Club" - and political - think "Iraq"), well-paced, and well-told. And Cameron simply takes movie-making to a new level with the innovative and creative use of motion-capture to create a new and believable reality.
And to those of us who scuba dive (I'm an instructor and I know Cameron dives as well), a lot of the weird creatures of Pandora have the familiar feel to us of anemones, featherduster worms, and jellyfish.