When I was walking into the theater to watch How to Train Your Dragon, I had mixed expectations.  On one hand, I was expecting a child's version of Avatar, on the other I was expecting to be seriously impressed. 
1. It absolutely was not a child's version of Avatar.
2. I was seriously impressed.
I saw How to Train Your Dragon in IMAX 3-D and it was truly amazing.   Sorry James Cameron, sorry Tim Burton, Avatar and Alice In Wonderland don't even compare.  The 3-D graphics were great, the story was great, and the movie was great.  It's an all ages movie.  I loved it, my daughter (2) loved it, and people in the audience applauded, I'm assuming that means they loved it. 
I highly suggest seeing it, with or without kids... and I ABSOLUTELY recommend IMAX 3-D.