My hubby, 5 YO son, 2 YO daughter and I really enjoyed the movie. It was a bit intense for my sensitive son, but most kids movies are. :D The story message was really great - appreciating what you have now. The animation was fantastic. The storyline was probably a bit rushed at times, but it is a kids movie and you only have so long to get to the point. I thought it was a good mix of humor, although the first two probably had a bit more (didn't see 3).
If you don't want to see it in 3D, you probably don't need to see it on the big screen. I feel that way about most movies though.