This movie was great... it's been a while since I've been to the theatre to watch something, and I had a blast watching Avatar. Will it change your life? No, but it's just 2.5hrs of pure entertainment and visual eye candy. This is definitely a movie you'll want to watch on the big screen, or at least enjoy on bluray. I highly recommend this movie to anyone who's looking for a good ride. However, the dialog was a little weak and unsophisticated... so don't expect to recite any memorable quotes from the movie. You will enjoy the beautiful mystical environments, action and sound... you will not enjoy the dialog.
Should you watch it in 3D? It didn't have any scenes that made you go WHOAAAAAA because it was 3D. You could easily pay less and watch the regular version and be just as happy. Nothing jumped out on the screen... though there were a couple of "ooooohs" and "aaaaahs" from time to time.
Overall, fun movie! You'll probably feel like this story has been done before though... for example, it feels like a futuristic version of something like The Last of the Mohicans from time to time.