This one's a weird one. Because of its general popularity I got some volumes of the manga and I enjoyed it well enough. Then I heard the novel was better so I read the translations. The story wasn't bad per se, it was just really really underdeveloped. Some parts get developed too much and some don't.
This author has a real bad case of tell don't show. It reminded me of Ai no Kusabi, another novel where the author has Tell Don't Show syndrome. For half the volumes we're just told Riki's, the main character, personality and just told about his feats of awesomeness without experiencing it. This is similar.
I get that it's a kids book with mature post-apocalyptic Utopian themes, but I felt that the author could've gone deeper into the society and their actions. For most of the novels we're stuck in Shion's and Nezumi's POV. Which is normal, they are the protagonists. But they never go anywhere or do anything for half the story. We just get brief snippets of narration that everything is not awesome. People who live at the bottom lose all morals- except not really. People who live at the top are gluttonous and evil- except not really. Man is bad! Except not really man, just one or two obviously mentally unstable people who inexplicably have absolute authority. Nature is good! Except not really, nature is just nature and humans are part of nature. In the end Shion and Nezumi interrupt their "terrible and depressing" lovey dovey slum lives, to go save Safu- the girl who loves Shion, that's her character. The world is saved, people have their morals- that they never really lost- back and Shion and Nezumi run off into the sunset...not. Nezumi has better things to do, except not really.
At the end of the day everything felt rather juvenile. Except for the last few volumes where things pick up, it's very slow moving and unfocused. We're just supposed to read it and naturally come to the conclusion that man is bad and nature is always right, but not all men, just the ones in power. Shion and Nezumi's relationship is sweet and they're the reason to read this story.
They're not here to move the plot along, the plot is there to move them along. I think most people know about this already. This is the "non BL" where two boys who are not family and totally in love with each other kiss but it's "not romantic". No, no those disgusting sexual feelings are way too impure for this beautiful non romantic relationship, where at least one partner turns down a willing woman he cares about for this man he's obsessed with. Shion may be "naive" but he knew Safu wanted his babies and very subtly told her "Not Interested" and subtly told Nezumi "I'm Interested". That's the way I saw it and it's weird that people see it as more complex than it is.