Well, seeing as how acid didn't actually play the game more than through the first level, I'll give you guys a full review of the game.
You're Jacky Estacado, member of the mafia, about to turn 21, when your uncle decides to erase you from "his" city because you've been causing him trouble. You go to perform a hit, find a bomb instead and boom you're dead....or at least you were supposed to be.
The Darkness awakens within you, and you get the chance to try it's powers out along with your revenge =D The story evolves from there, but I won't say more so as not to spoil it for any reader.
As for gameplay, the game plays really great. The aiming is a bit hard at first since you don't have a big target to aim at the opponents, instead you have a really small red dot a la Resident Evil 4, but you can configure how much auto aim you want, so it's not a problem. The game is much more than a simple regular FPS, you have a really big focus on storyline, a lot of dialogue, a lot of exploring, sidequests, etc.
You have a somewhat regular array of weapons, from dual handguns (your standard and most used weapon) to rifles and shotguns. And the really good point is your darkness powers.
You have 4 main powers: controlling one of your "snakes" to perform stealth kills, enter small places, open locks, blow lights, etc; Using one of your tentacles to kill opponents, lift parts of the scenery, blow stuff up and destroy lights; using two handguns that use your darkness powers and creating black holes that suck anyone near into them, along with half of the stage ^^'
Those powers make the game real fun and different, and along with them you become much more resistant because any attack or shot that you take while using your darkness powers (activated by pressing the assigned button) uses them up instead of your vitality, and also the ability to invoke darklings, small demon-like creatures, that come in 4 different types: berserkers, light-killers, kamikaze and gunners. The first type basically attacks anything that moves; the light-killers destroy lights and absorb electricity to kill enemies; kamikazes throw them at the enemy or any part of the stage that can be destroyed and blow themselves up and the last type use their railgun to shoot anything that moves :P
So all of this already shows how the game is really different from regular "shoot anything that moves" fps's, but then you also have the technincal aspects :P The game is really beautiful. Excellent lighting and textures, coupled with pretty good character models, great gore and blood effects, and spectacularly detailed stages give you a lot of eye candy to enjoy ;)
Then you also have a really good soundtrack, and excellent voice overs (too bad the main character barely moves his lips for some weird reason o_O).
Overall the game is really good (I personally loved it) despite some of it's flaws, which are, the 2 ones that I think should be noted, the fact that the game is small (15 hours is more than enough to finish it with almost all of the secrets) and that the sidequest have too damn much walking and too little shooting and killing :P Also the fact that you have those loading sequences in which you see jacky in one of his monologues EVERY fuckin time you go from one section of the stages to another really gets annoying after a bit.
But other than that, any action game and FPS lover should really enjoy the game. I'd give it an 8.5 score, out of 10.