I haven't got the DVD yet, but when I saw it at the cinema it frightened the living daylights out of me (guess I didn't help myself buy seeing it late night).
The thing about PA, for me, is it isn't full of gore and visuals to shock. It's psychological, it plays with your mind, because you don't know what happens when you're asleep. I'm not really a fan of the horror genre, but I think that not seeing something is scarier than seeing it..y'know?
Something Oren Peli said in an interview really struck with me; he said something like if he had hired someone such as Brangelina or someone well established, the audience would be more than aware it's this person, and their attention wouldn't focused on the actual narrative but more on the actor/actress, and so would probably miss something. I think that's why PA had such a big imapct I think, because Katie Featherston and Micah Sloat weren't known, it added a realism to it (as a girl in my cinema experience, when the film finished she said "Oh my godd! She might be in this cinema!"

)
I know PA has been a bit of Marmite-film, with people saying "It's not scary". You might not find it as scary as other films, because it's not like the typical horror film. It's a psychological thriller kind, which relies on mind games, rather than gore. But I loved it, even if it did terrify me and I didn't sleep that night.