I saw it two weeks ago and a woman practically ran out after the movie ended, she was so scared. Her friends were all laughing at her.
I don't think it was the scariest movie I've ever seen (it felt like a really slow buildup), but it was definitely one of the creepiest and it's completely worth it for the last 10 minutes or so, I still haven't completely recovered from that yet.
Best thing to do is see it with a big crowd that's really into it.