Darkness 2 improved over the first in almost every way and I'm happy to say this was a really good 1P campaign and the new co-op has good depth and variety almost like L4D. The cheesy mob story about Jackie and his darkness powers are still here, but the focus should be on the awesome gameplay.
Darkness 1 was all over the place, having you spend WAY too much time as these little dark minions (which was my biggest complaint). They completely stripped that away and totally beefed up your firepower with quad wielding. You have 2 serpant arms which can do serious damage, and then the option of using dual weapons also.
The depth and customization is here, and the different skill trees give you all types of variety with leveling up your serpants, fire power, new moves, etc. Picture ripping off a car door with one serpant arm and using that like a riot shield, now you have 1 more deadly arm and 2 guns to totally destroy everyone in your way. You feel too powerful at times, but that's the point.
There is also 4P co-op, giving you the option of choosing between 4 characters, much like a Left for Dead or Dead Island. The great thing is you get new XP to develop all 4 of these totally unique characters in this new game mode, called VENDETTAS. It's a lot of fun, since each character has their own special weapons and moves.
The worst part about Darkness 2 is the same thing that, for me, killed the first---The story is just awful. Jackie is supposed to be this bad dude with dark powers, yet he cries about losing Jenny over and over and over again. Like a broken record, he does everything he can to get Jenny back, even though we all know she's been dead forever. He cries like a girl and becomes a weak teenage boy when her name gets brought up anywhere in the story.
New game+ lets you replay with all your stats, so there's plenty of replay value too. The co-op mode exceeded expectations so I'd highly recommend D2.