Well, games like that really make you play till it ends. I won't spoil you with the details regarding the story because it's really complicated and puzzling and you have to play it to the end to understands it. I will tell you something about the gameplay. The gameplay is a classic FPS interspersed with some RPG traits. You can buy weapons, customise it and even improve your stats. You have plenty of enemies to shoot at and the powers with acquire in the game are really nasty ones: you can electrocute, burn or tear with the help of crows. Elizabeth,your companion throughout the game is pretty useful in opening locks and supplying you with health packs and salts.
The salts are the essence which helps you to use the "magic" powers.
I would give it a 10, but it's way too easy.