This movie does have some good acting by both Isabelle Huppert as Greta the stalker and Chloë Moretz as Frances the stalkee. Unfortunately that is not enough to save this movie from being disappointing.
After having lured poor Frances into her trap, Greta starts terrorizing both Frances and her flatmate Erica (Maika Monroe). The first half of the movie works okay, but the second half has problems. What happens is simply never believable and when it's not believable then it's neither shocking nor scary.
The script, the locations, the plot, none of these are good enough to make this a great movie. It's not horrible either, the actresses are too good for that to happen, but unless you are a big fan of Isabelle or Chloë you won't miss much, if you skip it.