I'm probably in the minority here, but I actually really like Darker than Black, in part for the positive reasons you mentioned (including the music and Mao the cat). I agree that the ending was bullshit, but I enjoyed the show more as an almost anthology series for the first 20 or so episodes until their attempt at having a proper arc at the end. Hei was obviously the main character of the series as a whole, but he was rarely the main character of any given two episode mini-arc, and I really enjoyed seeing all the different angles of the city and meeting so many different memorable characters (well, I found them memorable anyway. Especially the detective guy and his assistant, although their names escape me at the moment). The Cold War-esque noir tone coupled with the sci-fi and supernatural elements was really effective for me, and I think that most of the mini-arcs told good stories. I wouldn't call the series one of my all-time favorites, but I do like it a lot.