What makes the show is the main story and the characters. Off the bat, the two main characters show that a complex, turbulent but genuine relationship will form. Other characters introduced in the show are also fascinating with strange dynamics between them. I would have liked to know more about the world of the show. Like most future dystopian stories, there are things you want to know that both drive you crazy but also make the world even more fascinating. What No. 6 did right is focus on the story and characters so that you I didn't feel dissatisfied with the mysteries left behind.
The technical quality of the show is good but not the best. The animation was enjoyable but not incredible, same with the sound (though I really like the opening/ending themes). The dub is not the best either, but I still enjoyed it. To me, the character's voices matched pretty well.
Overall, I don't think of No. 6 as a masterpiece but I'd definitely recommend it, especially since it's short and doesn't have filler. And I'm always down for some good shounen-ai!
