Darker Than Black is a good show, full of cliffhangers. When I watched this, I had been out of the whole marathon-rhythm for a while, but I was still able to finish this in two days since I was constantly looking forward to the next episode.
While it had a high want-to-watch value for me, it wasn't anything incredible. While most of the side-characters were fun at first, some of them got a bit old later in the series.
It was a good action-fix though. Almost all of the fighting scenes were fun to watch and despite some arcs containing pretty much zero action, even most of these arcs had their strong points. Another big plus, the main character was awesome. I just loved how he was able to deceive people with ease, and while he almost has a flawless winrate, he isn't coming across as unbeatable fighter, even though he is incredibly fast and good.
Watch this as a dark action-fix, and you'll be fully satisfied by the show, but don't be expecting something incredibly memorable.