This is a fantastic experience! We saw Avatar in 3D IMAX, which is definitely THE way to see it. We took some family to see it less than a week later, and now those people have taken others to see it again as well.
Pandora, the alien planet, is spectacularly detailed, down to little gnats and plant variety. The Na'Vi culture is well thought out and believable as a real group as are the individuals. I never thought 'hey, that's a cool special effect'; it's just seamless and very realistic.
For such a well done movie, the plot could be much more complex and deep. Which probably means when it hits video, it won't be as popular. But I'll see it a third time!