Pros: Excellent story (good for both children and adults), interesting characters, unique dragon designs, shows consequences for actions
Cons: Astrid's character is underused
This is a wonderful coming of age story that will delight both children and adults - especially if seen in 3D. Hiccup is the son of a viking chief, but he is not your typical viking. Considered a disgrace by himself and his people, he sets out to prove he's a true viking by killing a dragon. What happens next is a magical adventure of self discovery and learning to understand the world around you, even if it's not the vision of the world that others share.
The movie was designed for 3D and makes use of that feature well, especially during flying and fighting scenes. When the ash begins to fall you want to brush it away from your face so you can see better. :)
And the dragon, Toothless, is very catlike in his motions. He makes you want to reach out and scratch him behind the ears.
Younger children may find some scenes scary. The theatre we were in had a lot of 8-10 year olds, who seemed fine. In fact, don't think I've ever been in such a quiet theatre.
The movie also has an excellent soundtrack.
*** Potential Spoilers ***
My one complaint with the film is that Astrid, the only competent youth in dragon hunter training, is given very little to do during the climactic battle. It seemed odd that all she's capable of doing is ferrying Hiccup to his dragon and then getting rescued by him when she falls off her own dragon. I'd hoped to see her fight alongside him.
On the other hand, I liked how, at the end, Hiccup does not survive the final battle without consequences. I think it's good for children to learn that doing the right thing can still have negative consequences.