Jennifers Body was written by the same person who won an Oscar for writing Juno. That's explains why some of the dialogue was pretty good. With the exception of Megan Fox I think most the actors did a pretty good job. She was good in some parts but then in others she seemed awful, at least to me, especially so when the others around her seemed to shine in their roles.
It dragged in the middle and in a few places toward the end but it was mostly entertaining and nothing like what I expected. Still, overall I liked the movie and I'd watch it again.