I'll admit up front that DD is a bit of a scardey-cat. (almost 3)
We went to see How to train your Dragon friday night. She LOVED the commercials and has been carrying around her McDonalds dragons for days. So, feeling optimistic, we went for it.
Bottom line, too scary. We were up and down and in and out of the theater the whole time. Glad that it was both DH and I so that we could take turns and each see a bit of it. The beginning has lots of scary dragon chase battle scenes/destruction, etc. It takes quite a while to get to the less scary "fun with flying dragons" parts, but no relief there, there's another big scary bit at the end.
I am certain that most kids will go flipping crazy for it and love every minute, but for DD, it was too much.
Re: How to train your dragon..
DS is almost 4 and he is super scary. He has never seen any movie in its entirety because he is just too scared during dark and/or suspenseful scenes.
He runs out the room crying and screaming during Cars when the truck is falling asleep on the highway (in the beginning of the movie). He runs out of the room or hides behind the couch often when he watches Dora (especially when there is a troll on)! There's no way in hell he could see this movie right now.