There's a lot of hype for this movie saying it's the "scariest movie ever". I don't think that's true but it's plenty scary enough. It's more fun if you go into it wanting to be scared. It's a little slow to get going but really pays off at the end. You'll probably want to punch the male lead by then, so it's sort of satisfying too what happens, lol.
I came out kind of laughing at the movie but liking the creepiness. I *did* have trouble getting to sleep that night :)
If you get motion sick easily from the "handheld camera" effect (like Cloverfield) you may want to skip though. My bf felt sick the entire movie. :/