Played it, couldn't complete it.
Though, my friend and I decided to have a Penumbra marathon and he managed to get through each of the games, except Requiem (don't even bother, it's just level-after-level of puzzle solving with no real plot). I have to say though, Overture was probably my favourite. Black Plague was freaky and mindforgetish, but still pretty loving scary.
Though the games are terrifying at parts, there will be moments where you'll want to scream at your computer because you can't figure out what the forget to do. And then when you do figure it out, you sometimes won't know how to do it. Every now and then you'll have to stop and think about what to do next, and actually read through your journals to get what is going on. You'll have to do a lot of back-tracking in it.
It's really fun to play at night, whether you're alone or not.
Penumbra <3