I enjoyed the Darkness for what it was. The story was nice and gritty and the atmosphere was well realised.
Not very re-playable though; I spent about 30 mins playing the online portion (after completing the single player) and never touched the game since.
For the sequel they should scrap the pointless online portion and concentrate on a tight single player campaign whilst making the guns actually useful this time.
There's more than enough online shooters to satisfy our needs. However an fps with no online segment these days is seen as a mule with no legs. Pity.