I went and saw Avatar yesterday. I never even saw a preview of this movie, had no idea who was in it (other than Sigourney Weaver), couldn't have told you what it was even about other than an alien planet. But I'm a huge James Cameron fan, and I went to see this movie on the strength of my faith in him as a director. Boy, was I not disappointed. This has to be the most beautiful and visually stunning movie I've ever seen. Holy smoke. The 3D was wonderful. It was never intrusive, never in your face, or used as a trick. It merely let you be a part of the world. I loved the characters, loved the world, loved James Horner's music, loved the sense of wonder the film evoked.... I haven't been this thoroughly grabbed by a modern movie since... er... I can't remember when. I can barely stand not heading back to the theater today to see it again this instant.
I don't know about your film experiences, but modern film audiences can be rather rude. Not in this movie. Nobody got up, nobody talked, nobody's cell phones went off. They were thoroughly attentive to the film and nothing else. I was sitting next to some young kids... you couldn't even tell they were there, they were so engrossed in the movie. The only time I noticed them was when they started bouncing in their seats and cheering at one point in the movie (an entirely appropriate moment; I felt like doing the same thing). They didn't budge otherwise. Can't tell you how refreshing that was.
Highly recommended. And definitely see it in 3D. In the theater. This is a movie that will never be the same on the small screen.