Darker than Black was launched in 2007 and has always been among the popular animes. I would say it is pretty good and one should watch it.
The anime in general is a little complicated to understand in my opinion. The concept of contractors, dolls etc is something one needs to pay a lot of attention to. The animation of the show is great considering it was launched in 2007. The storyline is really good although a bit difficult to understand in the beginning.
The action scenes are quite well planned out as well. The hero is a great character. He is adept in fighting, intelligent and a strategic genius. The story of various characters in the show is quite intriguing and keeps one glued to the show.
Overall, the anime is a great watch.