Short and Sweet
Darkness 2 is a fun, short, and very graphic first person shooter about a mafia boss whose bloodline is cursed by a demon called "The Darkness". God of War 3 is tame in comparison. What makes this game unique is a new game mechanic called quad-wielding. Basically you have 4 independently controlled limbs with which to attack at any given moment - 2 Human arms and 2 demon arms. For example, you can grab a guy by the neck, smack a different guy down, shoot one gun, and reload another simultaneously.
I beat the single player campaign in 6-7 hours, but there is also an online/offline co-op campaign called Vendettas with 4 additional characters that you can play through as. The Vendettas campaign took another couple of hours with one of the characters. One particularly cheap feeling encounter involving a helicopter made me not want to play through this mode again with the other 3 characters but I did missions with them in the "hit list" mode which consists of six 15-20 minute unique mini missions. There is also a "new game plus" mode, trophies/achievements, and a menu which suggests that new DLC will be available at some point. If you're on a budget or just in it for the story you can definitely move through most of the content in a weekend rental. If you're going for 100% I can see this easily taking 15-20 hours.
I do have a few complaints. A big bonus is that your characters keep their upgrades with every mission completed. The down side is that eventually you'll max out all of your characters making essence (the games currency) pointless. It's also not easy to tell which light source will hurt you or where the light is coming from at times, sometimes the boss battles can be a bit frustrating as you get bombarded with light toting bullet sponge enemies, and there doesn't seem to be an easy way to quickly discard/ignore empty weapons. Overall it's a solid, fast, fun game with good acting, badass combat and cool visuals. It feels really good. I can't wait to platinum this one.