This has almost everything that most low-effort manga is missing:
[ul]Beautiful, detailed art
A coherent theme and setting
A vibrant world built around a coherent theme and setting
Dynamic characters
Drama and tension. [/ul]
The theme is great: A child's loss of innocence on her way to adulthood. The plot isn't very intricate or dynamic: A little girl enters a bottomless pit to try and find her mother while also trying to figure out where her robot companion came from. But, the story is more about the journey than the destination. The narrative is character-driven rather than plot-driven. A simple plot provides a blank canvass for the author to develop the characters and setting.
My biggest complaint is all the gratuitous child nudity. It can be juxtaposed against how adults perceive nudity to fit the theme, but the author goes overboard. The manga is good enough to stand on its own without the need for fanservice to drive the plot or drive sales. It turns a manga that could have broad appeal outside of Japan to one that serves a niche market of fanservice.