I thoroughly enjoyed the game and I believe it had it's ups and down from the original. It was disappointing that they replaced most of the original voice actors, bar Mike Patton of course. It definitely wasn't as engrossing or in depth as the first one but there are two key points that I really liked about this sequel:
They made Jackie a more believable character in my opinion. In the first he was like a rampaging goth, which can definitely work (just look at the Crow) but I prefer the more mafioso Jackie in The Darkness 2. It could just be that I didn't like him being overly goth as he wasn't in the comics but I dunno..
I actually like that they are changing the comic plot. Most people complain when something is adapted into another format and it doesn't stick to the original but I've found with a lot of things, that somestimes a drastic change can do a lot of good. That way it's not like just playing some game of the comic/ watching a film of the book etc where you know exactly where it's going to go/what's going to happen. The game has become its own storyline in it's own right. Changing the way the Darkness is passed on (Jackie's dad still being alive after Jackie was concieved) and other points like how they've added the Angelus in etc.