Let's get the basics out of the way first:
* Go see it.
* Try to catch it in 3D.
* Unless your expectations are way out of whack with the kind of movie the trailer advertises, you'll greatly enjoy it.
* It has flaws, but they don't ruin it.
Now, let's get to some specifics.
The look is as awesome as you've heard. I've enjoyed many of the new crop of 3D movies, but
Avatar is by far the best of them at using 3D well. Almost all of the time, Cameron integrates 3D into the shots, so it adds life and realism to the movie rather than serving to distract or simply screaming, "Hey, look, a 3D shot!" Even without the 3D effects, the movie is simply gorgeous. No alien world may ever prove to look like this one, but this is the way we'd like an alien world to appear.
The story is basic and predictable, but I believe that's intentional, because in
Avatar Cameron is all about myth-making. He tells us so repeatedly, and the story operates best on the mythic level.
That said, the acting is better than most critics claim. Sure, some of the characters are too easy, especially the bad-guy military leader, but even he is wonderful to look at and so reeking of his character's single-minded focus that we don't tire of watching him.
Perhaps as important as all of those things, however, is one simple fact:
Avatar is a game-changer that will become the standard to which we compare SF movies for some time. In the same way that
Star Wars and
The Matrix showed us things we'd never seen before and reset our expectations,
Avatar raises the bar yet again and makes us realize just how good modern SF movies can look.