Non-spoilery version: There's no denying that this is a distressing movie. I found it almost as distressing as Perfect Blue, which is quite an achievement in a PG-rated film. However, I think it's a great movie, for reasons very similar to why Perfect Blue is one of my favorite movies.
No more spoilery than a normal film review version: Spike Jonze has made a fairy story for grown-ups. It's easy to forget what fairy stories are like for small children - as we grow up, what we remember is that we found fairy stories comforting. And they are comforting, once you know how they end. But the biggest fear small children have is that something will happen to their parents, and fairy stories tend to begin that way. As a parent, I've told fairy stories to small children, and become re-acquainted with the terror that they induce.
The first time you tell the story, you have to stop several times to reassure and to explain. Only when you're telling it the third or fourth time can you roll straight through and tell the story from start to finish in a clean motion, because only then has the child developed the understanding that is required to cope with the distressing parts of the story. This is what makes fairy stories worthwhile as educational tools - what starts out as unbearable without help becomes bearable and then becomes comforting. The world that seemed so incomprehensible and impossible becomes something easier to understand and easier to bear.
The fairy stories that really mean something present something truly terrifying, and help towards seeing that horrible thing in a better light. It's not easy to see the positive in the film version of Where The Wild Things Are. Sitting there, surrounded by parents and their kids, I wished I was watching it with them, and imagined myself helping them cope. In the end, that's the reason I was able to cope with it myself; having the 'parent mode' of my mind activated meant that I had someone explaining the movie to me. It was terrifying and difficult, but ultimately reassuring. (The fact that I already knew how it ended really helped.)
The book of 'where the wild things are' is the story of a misbehaving child. Max is very badly behaved, and is sent to his room. In his room, he goes on a fantasy journey to an island populated by monsters. He conquers the monsters, rejoices in his imagined power, calms down, and rejoins his family.
The book presents a very real and very distressing childhood problem (being sent to your room is a very upsetting experience), and gives a reason for hope (you can rejoin your family when you've calmed down).
The movie is more ambitious - it presents a very real and very distressing problem for people in general (people hurt and injure each other), and gives a reason for hope (people can support and heal each other).
In the movie, the story begins with Max being an adorable and imaginative young boy, who is doing what he can to have fun, and share his fun with others. But his teenage sister doesn't want to play make-believe with him, she'd rather play with teenage boys. The teenage boys are willing to play with him, but they don't have well-developed self-control and risk assessment skills, so they end up hurting him in a more direct and physical way. The sister, aware that her little brother isn't seriously hurt, doesn't pay much attention to his distress, and Max responds to this injustice by taking revenge.
By the time that his mother comes home, Max is aware that his revenge has achieved nothing, and together they repair the situation as best they can. Some things, once broken, will stay broken, but that doesn't have to mean disaster.
However, Max has not been able to resolve his loneliness and restlessness. His mother (tired from helping with repairs, and from the demands of adulthood) is unavailable to meet those needs, she's busy getting the support she needs to endure her loneliness. So Max switches from 'good cop' back to 'bad cop' and tries to use force to get what he wants from his mother. This confrontation leads to a perilous journey across the sea, to the island of the monsters.
Max immediately forms a kinship with the monster who is expressing his rage (the monster's family has abandoned him) via destruction. There are other monsters, too - a monster that tries really hard but is always ignored, a monster that insists on being in control all the time, a monster that seeks safety from being controlled, and a monster that is wise and considerate but isn't around very much, and doesn't seem very interested in solving the pressing problems of the moment.
Max becomes the king of these monsters, and does his best to be a good king, caring for his subjects and trying to meet their needs. But it's harder than it looks.
Things reach a crisis when Max repeats the acts that started the movie - injuring someone because he is unaware of how much power he has, and failing to meet someone's expectations because he's changed his mind about what he wants. The monsters respond the same way that Max did, with revenge and violence. The futility of revenge and violence is made clear, and an alternative is presented - understanding is shown as a kind of strength that allows us to end the hurtful cycle.
Max decides that he wants to go home to real-life family, even though he still loves the monsters. They are sad that he is going (which is a kind of pain) but they understand (which makes it bearable). He returns to his mother, who demonstrates that she cares about him.
People hurt each other; there are ways to avoid this, but it can never be truly prevented. However, hurt and injury can be survived, and the journey through hurt and injury can lead us to understanding and strength.