Puzzles?
Hmmmm... not very difficult. Take this thing here... jump over there... dismember this alien...
You play it for the creep factor, rather than the puzzles. You can complete the story in a few hours (six, perhaps???). Depends on how you play it. If you play it in the creepy spirit it was meant to be (by slowly looking round corners and generally watching your ass), it'll last a little longer. More if you use a higher difficulty setting.