Summon those cheeky Darklings
The main power of the Darkness, and the one which you'll be using most frequently, is 'Darkness vision', which allows you to see darkling gateways from which you can then summon these beastly creatures. The Darklings have glowing eyes, sharp fangs and claws that can rip through human flesh and overall there are four Darkling types to experiment with: Berserkers, Kamikaze, Gunner and Light-killers. Whenever you see a gateway, which are littered all across the environment and take the form of a small portal in the ground, you can choose the type of Darkling that you wish to summon. You can't play from the perspective of a Darkling, but you can tell them where to go by pressing the X button and they do become tactically important the further you progress. I'll give you some examples as to their use: A Berserker Darkling will attack any enemy in the vicinity, he'll jump on them and bite them to bits leaving them for dead, and he'll also solve puzzles for you, such as clambering over a wall to unlock a gate to the next area. They can also be used to distract enemies so you can reach a certain position.
The Kamikaze Darklings are great fun to play with, they have dynamite strapped to them and if they walk close to an enemy they'll blow themselves up, taking out anyone around them. The Gunner Darkling wields a huge chain gun and when you send them forward in battle they'll shoot at any enemy that gets in their way. Finally the light killer sends out an electric charge from his claws taking out any light sources in the vicinity.
Using the Darklings is great fun and there are so many options open to you in the ways that you wish to use them that there's no one particular way that you can tackle enemies with these creatures, there are often multiple ways to kill an enemy. If I've got one complaint, then I'd say that their path finding skills need to be tweaked slightly before launch because as it stands they don't always go exactly where you tell them to.
One thing I mentioned earlier was that you'll be devouring hearts, which is something you'll be doing often in 'The Darkness' as it allows you to rise up the ranks of evilness and unlocks greater powers for you to use. Whenever an enemy is dead you can manifest into 'The Darkness' and rip out their hearts with your serpentine tentacles and munch on them like tasty tomatoes (what an awful comparison). The more hearts you eat the quicker you'll rank up to the next level where you'll be able to cast abilities such as the Black Hole, which can suck multiple enemies into it, dragging them into the depths of hell.
The pace of the gameplay in 'The Darkness' changes frequently between combat and slow bouts of adventuring and information gathering, in which you'll walk around interacting with characters, picking up missions and side objectives. The blend of fast paced action, which can manifest at any time, and the slower missions is a nice balance that stops the game from ever feeling repetitive. There are some bizarre, but entertaining objectives such as when an old lady throws 5 coins onto the train tracks and invites you to pick them all up before the train arrives or when you need to find some thugs who've stolen a harmonica from a tramp who frequents the subway. You could just progress through the game straight from mission to mission, but there's a certain freedom to the gameplay allowing you to wander about areas of the city picking up these side missions and picking up collectibles, which in turn unlocks content from the original comic book of which 'The Darkness' is based on.