Animation has finally been done right--there's a real plot, it's funny for the grown-ups and safe for the kids (as opposed to some of Shrek's questionable choices). If you're the type to find them and love them, there are some cool messages in the plot. If you're the type to enjoy a movie without thinking too much, it works. It also gives parents a whole new spin the next time they have to banish monsters in the middle of the night.
Review of Monsters, Inc.
