Story: No. 6 takes a story that could have been cliche and makes it interesting. The world set up within the story seems like the overly controlling society often associated with "1984." I was shocked to find that I enjoyed the direction the creators took it. I watched these episodes every week as soon as they came out because I couldn't wait to see the next instalment.
Animation: The animation has been well done. It is solid and skilled but isn't anything new or unusual. It definately isn't a show where the animation stands out in a huge way.
Sound: I am, personally, not a fan of the opening theme song. Other fans tend to like it more than me but I find it irritating and it tends to grate on my nerves. The closing theme, however, is completely lovely. I think the voice actors did an excellent job. There were scenes where just the despair in the character's voices made me want to cry. They were truely great with one exception. They used the same voice actors who voiced Sion and Nezumi for their adult and child selves. Thus, the child versions of these characters end up sounding far too old.
Characters: I was very attatched to the characters from the first episode. Sion is a very sweet character who is a little bit unusual. He may come off as a bit naive at times, but I think that this fits well with his character design. Nezumi is a street-wise, independent guy. There is a really vengeful nature to him that I enjoy as much as his caring side (which he shows occasionally yet reluctantly.) There is a light shonen-ai aspect to their relationship that potential viewers should be aware of. Their relationship was a tender back story to the destruction around them. I personally thought the relationship was sweet but I do think it is a fact worth knowing before you start watching it.
It was not perfect and had quite a few flaws, but that did not stop me from thoroughly enjoying myself.