The manga comments on and shows us about lives of people who are physically very different from majority of us. Very truly, without any softening they show how their lives are. What they have to do to survive. As it shows harsh realities, it is no manga for children as the bitter reality will be too much for them to digest. It is not obscene, just states the things as they are.
The manga also has elements of fantasy in it. But those things haven't been very well explained. But I may be feeling that because I am an outsider to Japanese culture. It also has heavy nostalgia towards the end. The feeling of nostalgia doesn't sit very well with me, because I think missing the past doesn't do anything much for us, doesn't serve much of a purpose. But my belief doesn't void the fact that we all sometimes miss the past. Even I do.
The manga also raises the question of how things could have been very different from how they are. But that remains for reader to answer. For the plot, as well as in generally.
It is a good manga for serious reading. I would recommend it.