Roger Ebert mischievously described it as 'Twlight for boys', and this is really where its problems lie. It wants to retain much of the indie cleverness of Juno whilst ceding to the genre conventions of high school horror...rather like a jock that wants to dress like the EMO set off the football field. (Or Cody herself doing a cheerleader routine!)
Presumably this need was driven by the requirement of extending the financial power of the Diablo Cody 'brand', but the results are hardly encouraging.
Cody throws everything at the screen — sex, repartee, gore, incipient frights, genre references — but nothing really sticks.
There's one short passage of very clever and funny dialogue (tellingly the one I'd heard as a clip before watching the film) but it can't save the rest of the screenplay from patchiness: sometimes the characters are in character and sometines they are mere mouthpieces for Cody.
Having thus far avoided the Transformers movies I haven't had an earlier opportunity to form prejudices about Megan Fox, so I can't really say — as did Mark Kermode — that this is her finest work to date and most probably the finest work she is ever likely to deliver as an actress. But in a movie which is ultimately a failed attempt at crossover, her own crossover (and upgrade to a lead role) is possibly the least unsuccessful.
Update: Fox explains the film's poor showing at the box office thus: "Jennifer’s Body’ wasn’t rated PG-13 like Twilight...It was a hard R, and kids couldn’t get in. So they bought a ticket to another movie and snuck in." haha..yeah right.