Excellent game! It has more a traditional FPS feel than the original Darkness (which has more of an open world feel to it), but the gameplay is for the most part sublime. "quad-wielding" is awesome, and the executions are epic (though those can get old seeing the same one over and over for the 100th time...to some people lol). I will say I liked the voice acting better in the original, but other than that I'm leaning towards the second one being my favorite of the two overall.
BTW I know the main campaign is short, but there's also a "Vendettas" campaign where you can play one of four of Jackie's hired assassins. It's pretty fun, and adds a few more hours to the game in total. And the Hit List too, which takes missions from the Vendettas campaign and mixes them with a few brand new ones (those are mostly just killing guys and clearing out areas).
Anyway, for $20 I'd say it's totally worth it. I'm playing it right now and loving it.