There has been some mega-buzz about Elizabeth Olsen's performance in this film, and I can argue with confidence that she is a more talented actress than not only Mary-Kate, but also Ashley. Couldn't resist. In all seriousness, she is excellent in this film. I didn't enjoy the movie as much as I thought I would, and I think it's because I just had no idea of the genre. I had heard that the plot involved a girl (Olsen) trying to readjust to life with her older sister after some time spent in a creepy and abusive cult (as opposed to a breezy and normal cult) mind-controlled by Patrick, an also excellent John Hawkes.
But whereas I expected a character study about the nature of trauma, what I got was a psychological thriller. The title character--who was Martha pre-cult, Marcy May and Marlene while en-culted--experiences not only paranoia and flashbacks, but also becomes a dangerous figure herself, acting out violently and inappropriately against her well-meaning sister and, to my mind, not so well-meaning brother-in-law.
I had issues with Martha's character (e.g., the film didn't do enough to convince me that the two years she spent in Patrick's thrall would result in the sort of bizarre behavior she exhibits) that I might have excused if I knew I was watching a horror movie. Similarly, I found the cult a pretty unbelievable character, if that makes sense. Its internal logic felt uneven and jumbled to me, and though the different operating principles of the group work for a horror plot, I think one thing cults have going for them is consistency of message. This one wasn't shown to have that.
And I'm not sure my ambivalence is entirely the fault of my ignorance. At times, the film seems deeply interested in the dynamic between Martha and her sister, the familial past that produced two very different siblings, and the reason Martha was seduced by Patrick, and also why she misses him. But in order to amp up the tension, those, to me more interesting, elements are dabbled with and then ultimately dropped to service the horror plot. Martha Marcy May Marlene is good, but know what you're getting into.