"IMAX Experience" is not IMAX. In my theater it was just a bigger screen. I would recommend you skip "The IMAX Experience" altogether.
The 3D was as good as advertised. Just enough to give the characters a 3 dimensional look. Try to sit as close to the center line as possible.
The Animation is the best I have seen, the focus on facial details in close-ups makes the characters real and you forget that you are watching computer generated characters.
Plot and character development is everything in a movie and AVATAR is an exceptional movie.