In the world of "no good deed goes unpunished," the story warns us not to take in people in need or they'll wind up eating your cat, your neighbor, a carnie and your new boss's son. But that's okay because she'll also have awesome sex with you. As morals go it's a bit dubious, but Jenifer, to its credit, is not a moral tale. It's more of a tragedy of bad choices, and that's something I can get behind. Too bad the story itself is so mediocre.
The saving grace here is Carrie Fleming. Her performance as Jenifer is fearless, interpreting her as a mix between a wild animal and an innocent child. She brings a lot of raw emotion to a role where she doesn't have any dialogue outside of shrieks, grunts and wails. At least she brings her A-game, even if Argento doesn't. And Weber is his usual wooden self, relying more on stubble and makeup than acting skill to indicate his downward slide.
Also, the ending will come as no surprise to anyone who has ever read a book or seen a movie, ever.