I wasn't too sure how good it would be after seeing the previews. Frankly, I found them rather disappointing. They indicated that the movie was quite different from what I was hoping from an epic Cameron Sci-Fi movie, and not in a way that I liked. I have to say, Cameron's "Aliens" is easily among my top five favorite movies of all time (and I'd have a hard time naming four that I prefer on the top of my head). I had little hope that Avatar would reach that high.
Now, I still didn't like Avatar as much as I had hoped. But it was better than I expected. The visuals are absolutely amazing. It's really a beautiful movie. However, the plot is very standard stuff. I still feel that Cameron's masterpiece is Aliens. It's pretty much a perfect movie. Granted, the general plot is fairly simple, but its various elements flow together so well. I didn't expect Avatar to be nearly as good as Aliens, and I was right (according to my own personal tastes, of course). But it was an impressive show nonetheless.
I do have doubts about sequels, though. Does Avatar really needs sequels? What more is there to tell? Having more Earth people come to try to take Pandora again would be so lame. Maybe the Padorans will try to invade Earth for its resources, how's that for a really silly twist? In any case, a sequel wouldn't do anything to continue the plot. It would just be a different, unrelated adventure of the Pandora people. Kind of like the Ewoks movies (yuck!)