Overall the film was very entertaining. Lots of action and surprisingly funny. The twins and RC stole the movie, with the twins having some of the best lines.
I was surprised at some of the language in the movie, not that any of it offended me, just surprised. For a film with an appeal to children under 13 I didn't expect it.
As far as IMAX is concerned, save your money. I did not see any significant difference in IMAX.
2 1/2 hours is a long time to spend sitting in a movie theater. The movie had good flow throughout and kept the viewer involved. But 2 1/2 hours is too long; add that bucket the theaters call a "medium" drink to the mix, and biological needs quickly arise.