The whole family loved it, including an 11-yr-old and a 7-yr-old. Beautifully animated, especially the dragon (like a cross between a Stealth bomber and your cat) with good characters throughout. A couple of unbelievable plot twists, like Astrid's sudden conversion to dragon-lover, but really who cares when the rest is that much fun.
I thought the movie was wonderful and very true to the spirit of the book. My six-year-old loved it too - I think he thoroughly identified with Max and his troubles, and the monsters are so engaging that it doesn't matter if kids don't understand all the finer points of the plot. They understand the raw emotions being shown by monsters who talk like kids.