This movie stands on its own as a touching story about a boy learning about the pain he is able to inflict on others by exploring the possibilities using his own imagination. Rather than turning the book into a whole different story, it lifts it up and expands it into a fully fleshed-out story about the struggles one boy can have trying to learn how to deal with the emotions and creative power he has in the real world.
After unintentionally being either hurt or ignored by his family and their friends, Max runs away to an island full of monsters that make him their king. As he learns the intentions and the stories of the monsters, he begins to understand both them and himself. The first act with him at home dealing with his family is touching and heart-felt. If the rest of the movie had followed the first act's lead it would have been a masterpiece. Unfortunately, as soon as he gets to the island the movie seems to unintentionally lose focus. It's easy to say that this is because of how difficult it is to translate a ten-page book to a 90 minute movie, but the plots that are set in place should easily compensate for the time expansion. Rather, I think it's because the main plot is never quite as easy to recognize as the subplots.
Before I go into spoilers, I just want to say, that this is a movie that any fan of movies should see. At no point during the movie did you ever think that the monsters were fake. The combination of Jim Henson puppetry and CGI facial enhancement was superbly implemented and this along with the design, look and feel of the entire movie makes this a beautiful artistic achievement. When I say it loses focus, I don't mean for that to deter you from seeing it, just as fair warning. Where the Wild Things Are is a perfect movie to see as an adult reflecting on what it was really like to be a kid.