I saw it in 3d as well, but in RealD, not IMAX. From the reviews I've read, and my own personal experience, RealD makes for a much better viewing experience. The glasses aren't those goofy red and blue paper things, you can view the movie perfectly even if you tilt your head (unlike IMAX), and the movie appears to have more depth to it, rather than pop-out images.
Aside from that, the movie was awesome. I'd definitely go see it again. The animation was spectacular, colors amazingly vivid, awing graphics, and a typical-but-still wonderful plot.