I'm recovering from Christmas lasagna. So this will be Q&A style.
Is the actual film better than what is presented in the trailer?
How are the special effects?
-Absolutely amazing. Almost worth paying 13 dollars for a single movie ticket. No uncanny valley with the humanoid characters, and everything else was breathtakingly well rendered.
How was was everything else?
-Incredibly average, even clichéd. The story is basically Dances with Wolves in space and the characters don't really develop their standard action movie tropes. Basically, this is exactly the kind of storytelling you'd expect from a James Cameron movie.
So it was bad?
-Not objectively no. I'm really tired of these "going native" stories. Although this is less offensive than say The Last Samurai, its really just another "white guy hangs out with non-white guys, learns from them, eventually gets accepted by them, and then eventually has to save them" kind of story. However, all James Cameron's movies are basically standard plots with some awesome special effects on top. Heck, the whole overzealous military plotline is almost identical to the one in The Abyss. With all that in mind its hard to say that Avatar is bad, mostly its just kind of dumb.
What about the action sequences?
-Say what you want about Cameron's tendency towards well worn plots and stock characters, but he certainly knows how to put together some action. Putting together a coherent and engaging action sequence seems to be a dying art, but wow is there some breathtaking stuff in this movie. Mecha vs. dinosaur fights played completely straight and somehow it works out and even seems believable.
So is it worth seeing?
I think so. Mileage may vary depending on your tolerance for well worn plots and action movie violence.