Short Version: Overall Jennifer's Body is just a boring movie - punctuated by occasional humor and fewer scares.
Screen Rant reviews Jennifer's Body
Let's just get this right out of the way: Megan Fox does not appear naked in Jennifer's Body.
So if that's the main reason you want to go see this film, you're out of luck. If the other reason you want to see the film is for the girl/girl makeout scene - there is a very brief scene of a kiss between Megan and Amanda Seyfried, and it's shot in such an extreme close-up that for all I know they might have had "stunt doubles" do it.
Now that we have that out of the way, on to the review. :)
Megan Fox plays Jennifer, the uber-hot, like, totally popular babe at high school that girls envy and boys want desperately. She's a typical movie hot-chick high school cheerleader, caring more about parties, guys and what she looks like than things like schoolwork. Her long-time best friend is Needy Lesnicky (played by Amanda Seyfried, obvious pretty-girl-hiding-behind-glasses-bad-hair-and-bad-clothes). It's obvious quite early on that their friendship is not exactly balanced, with Needy cowtowing to Jennifer in every way, but happy to do it due to a misplaced sense of loyalty and the fact that being so UNpopular she of course gets a thrill out of being the popular girl's best friend.
Jennifer being Jennifer, she gets into groupie mode over the lead singer of a visiting band (played by Adam Brody). She persuades Needy to ditch her "good guy" boyfriend Chip (Johnny Simmons) and to go with her to the only bar in town to watch the band play. By the end of the evening, Jennifer ends up taking off with the boys in the band, who are obviously not exactly Boy Scouts. Later that evening Jennifer turns up bruised and bloody at Needy's house, acting strange, creepy and threatening.
Soon thereafter a series of grisly murders invade the small, usually quiet town, and Needy starts putting two and two together. Of course her boyfriend won't believe her when she tries to tell him what's going on, our main characters end up in grave danger, etc., etc.
Jennifer's Body was written by Diablo Cody, the woman who wrote the critically acclaimed Juno - so despite the fact that this film was starring Megan Fox, people were hoping for something worthwhile. However where the dialog in Juno flowed easily and naturally, and the wit came easy, here it all seems forced and you get the sense that they're just trying too hard to be hip - and failing.
You want an example? How about "They're agents of Satan... with cool haircuts."
Oh, hang on a sec - I've got to step away from the keyboard due to the non-stop laughter that won't let me type.
The movie actually starts to get interesting once the creature appears and the killing starts - actually that's when it starts to get at least a bit funny. There were maybe two or three times that I laughed during the entire film. So OK, but is it scary, then? No, not really. Right at the start of the film there is the weakest "jump scare" I have ever seen in a film, and while there are maybe a couple of horror movie-worthy scenes, overall it doesn't work on that level either. I will admit that the smile that Megan Fox gave when she first showed up covered in blood DID creep me the hell out.
There were a few laughs here and there, one early on when Jennifer heads off in the band van and one funny bit during an awkward teenage sex scene (no, it doesn't involve Fox, sorry).
And then there's Megan Fox. For the guys, this still from the film shows more of her than you'll see in the entire movie, so you can just get your fill right here:
Frankly, based on what I've read about Fox and from her in the form of interviews, it seemed to me that the character she was playing was pretty close to just being herself - not much of a stretch. The Jennifer character seemed so close to reports of those who've worked with Fox, and based on the sort of statements she herself has made - I just couldn't buy into the character. I just saw Megan being Megan (except of course for the whole fangs and eating entrails thing).
Highlights? J.K. Simmons appears here and once again plays a slightly off-center supporting character. Love the man's delivery of even the most basic lines of dialog. Amanda Seyfried did a nice job as the nerdy protagonist, and Johnny Simmons was convincingly sincere. And Adam Brody did some scenery chewing in his brief scenes that made it look like he was having fun with the role.
As Annalee Newitz of io9.com pointed out, Jennifer's Body is about the horror of abusive female friendships instead of the more common female sexuality angle - and it's commendable that writer Diablo Cody and director Karyn Kusama wanted to try something different... it's too bad that it just doesn't work.
But hey if you just can't get enough of Megan Fox on the big screen and acting sexy, supposedly hip dialog, awkward teenage sex, a bit of gore and mild scares, then maybe this movie's for you. Me? I was bored and for a 102 minute movie it felt WAY longer.
Oh, if you do go, stay through the credits for some "closure."