Lame. There's a lot of untapped potential here in this story literalizing the hell of high school social relations into an actual story of demonic possession, but what ended up on the screen looks like it was about three screenplay drafts away from being ready to shoot - the character relationships are underdeveloped, the structure is clunky, things are set up and then poorly paid off, if at all.
I'll give the filmmakers credit: Diablo Cody's script does have its share of good dialogue and jokes, and the cast does a good job, especially Amanda Seyfried in the lead and Johnny Simmons as her boyfriend (and the rare actor in a teen movie who actually looks like a teenager). Even Megan Fox isn't awful as Jennifer, showing a lot more acting ability than Michael Bay ever bothered to try and get out of her.
The problem is that the movie's emotional connections don't connect; we know that Jennifer, the school's hot slutty bitch, has a strong, co-dependent relationship with Seyfried's geeky character Needy, but the relationship is skin-deep - the movie's idea of investigating these characters is to give them a gratuitous kissing scene. Not that I'm against girls kissing, but Cody's screenplay does such a poor job of building up to it, or tracking the follow-up, that it really serves no useful purpose (beyond the obvious).
On top of all this, Karyn Kusama directs like a first-timer, with no sense of space or mood. But my biggest complaint is the presence of Adam Brody as the lead singer of a band who sell their souls to Satan in exchange for success. Yeah, they're responsible for turning Jennifer into a demon, but their connection to the story is so incredibly poorly managed that they really distract from everything else. Okay, my BIGGEST complaint was the huge number of plot holes, but I could just keep going on forever.
So in conclusion, what Jennifer's Body tells me is that Jason Reitman was the unsung hero in making Juno coherent and emotionally engaging.