If you work as a filmmaker or in television, whether as a hobby, your profession or your obssession, YOU NEED THIS BOOK. Screenwriters for both film and TV especially need this, since it deals largely with storytelling cliches, but it also lists visual ones in cinematography, in angles, in casting and in general mise-en-scene that it is absolutely crucial for the director to avoid. This book will make you a better filmmaker just on virtue of being aware of what's been done to death.
It's also useful across the board. While it usually rips into the more standardized genres (like slasher flicks or action movies), it also chainsaws such common cliches as "The Pet Homosexual" ("he can talk endlessly about sex, provided he never has any himself", most recent offender: "The Next Best Thing" and "Will and Grace"), "Baked Potato People" (the gentle lunatics in the asylum that show the outside world is crazy; most recent offender: "K-PAX"), and more subtle ones like the Fat Guy rule; if a group of men are planning an escape, the fat one usually can't be trusted.
This is a very funny book, but it's also very true, and if we made everybody currently making movies sit down and read the damn thing, we'd have better movies, or at least different cliches. Fun for the armchair film freak, but absolutely crucial for the filmmaker.