I went to see Avatar in 3D this weekend, and now I finally understand what is so impressive about this medium. The movie was so lifelike, and contained such depth, that it seemed like you were really on Pandora. Kudos to James Cameron, who chose not to throw the 3D in your face. None of it was gratuitous, and all of it worked.
Here's the trailer, but it doesn't do the 3D version justice: