saw it yesterday with my son and loved it (altough there is the danger that the movie appears to better than it actually is, because of -> movies you watch with your kids become better movies).
Seriously, How To Train Your Dragon is a better movie than Avatar, not only because of the great 3D effects, but also because of the less predictable script... and it was really amazing to see that the boy XXXXXXXXXXSPOILERxxxxxxx (in a kids movie). I thought that was a really great detail. The kids in the audience were stunned/shocked to learn that XXXXXXXXXXSPOILERxxxxxxx (some "R" rated movies should take a close look..)
If a movie is great, 3D does not hurt the experience... and HTTYD was a great movie with 3D... 3D is okay for action-packed event movies, but would it be out of place in a movie like The Messenger?
The Fall could be great in 3D, I would not mind to see it in 3D...