I finished No. 6 a couple days ago, and I was not overly impressed. It left me with a, "That's all?" type of feeling, and I felt that too much vague background story was rushed through in too few episodes, and I was left with many unanswered questions, most notably, "Is Nezumi male or female?" Unfortunately, it didn't hook me enough to want to read the novels (if they are even translated).
I will say, though, that I did like that No. 6 revealed information through the story and characters' actions and beliefs, instead of "telling" via long, obnoxious monologues and ill-timed flashbacks.