I've seen Avatar two previous times in 3-D, four or five times on DVD showing it to friends, and now the new special release in 3-D Imax.
I have to say that I appreciate each viewing experience for what it has to give me. In the original 3-D version I appreciated being able to experience the movie's eye-candy in a smaller theater with decent seats. At home I could watch the movie on a widescreen set and enjoy the very "Dances with Wolves"-like story. On an Imax screen I found my gaze being drawn to different areas of the screen, noticing details I'd previously missed.
That having been said, although none of the added bits changed the story significantly, they worked together to clarify or enhance it. Not only do you get more delicious eye-candy, you also get an explanation of why the corporation attacked the tree of souls, and how Sulley became the leader of the Na'vi.
If you enjoyed Avatar before, treat yourself to this extended release and I'd say chances are you won't be disappointed. If you didn't like Avatar the first time, this won't change anything. Spend your money and your time elsewhere. :)
Oh, and the three year old sitting in the seat beside me kept trying to catch the little seeds and flying critters. He was almost as much fun to watch as the movie. :)