A great scary game that will suck you in!
The only negative aspects points on this are:
-the game is to short
If you play Penumbra, you wont feel to stop, so you will eventually finish the game in a few hours.
-some puzzles are a bit hard to know what to do
You must think, but it doesnt mean you have to be Einstein. Theres a few (few) puzzles that you will stuck and you wont know what to do next, or...not. That depends on you, but this is the opinion of more people out there.
-theres some bugs to avoid on the game.
Theres some bugs, related to things you can do, but should do, or the game will crash. eg: entering a room thats "locked" but you manage to open the door. Doing this may crash the game if you collect the "important tools" in the room and use them...when you weren't supposed to use them! I had that problem.
Despite the bugs, Penumbra is a great game, and in my opinion, it deserves a 9. So try it, and enjoy it!