It would be hard to make a boring film with such an impeccable premise, but Teeth entertains on many unexpected levels.
For example, could a more perfect choice have been made for the bearer of Vagina Dentata than teen abstinence warrior Dawn? The writing is tight enough to keep you speculating about whether or not she realizes that anything is wrong with her inner labia...perhaps she is preaching abstinence and the "dangers of sex" because she actually thinks sex is really, physically dangerous.
A censored vulva in health class further compounds the issue, as well as a ubiquitous factory pumping out fumes in the background of every other exterior shot and some discussion about evolution and mutation. Dawn visits a gynecologist to make sure she doesn't have any "adaptations"(!!!). There's also a stepbrother who may or may not remember how his finger was nearly severed during a session of playing doctor (another brilliant detail: he avoids his sometime-girlfriend's vagina in favor of anal love).
What is unexpected is how fucking SEXY this movie is, even after the knowledge of impending dis-memberment (a pun I stole) is well established. Dawn's budding sexual fervor is vivid and believable, and going from ultra-virginal to having her breasts fondled and clitoris vibrated (and ENJOYING it tremendously, date-rape drug notwithstanding) struck a titillating chord with both male and female members of the audience. I love this girl, her performance, her girl-next-door cum sexual predator character.
The rare "comedy/horror" that is actually scary (connecting her vaginal condition to pagan lore, the discussion of her tooth's physiology), and actually funny (her stepbrother brings up her abstinence: "We all know who you've been saving yourself for, and I've been more than patient"), Teeth is every bit as delicious as you want it to be.