The background is very, very interesting: God has abandoned the world. No more births, no more deaths. So if you die, you basically keep on being yourself -- only a grave dug by a 'gravekeeper' will allow you a peaceful, eternal slumber. And everybody's secret wishes are granted as well, which gives some people special abilities, or twists the world around them.
We follow the adventures of Ai, a girl who is half gravekeeper and half human. She is the youngest person on the planet (being born after the 'last generation'), which makes her a curiosity. With a rag-tag band of humans and another gravekeeper, she travels around and finds herself in all sorts of situations.
The background is filled with interesting stories, but we only get to see a few of them. Some are really great (such as when Ai and her band visit a city that is populated exclusively by the dead), but others have a much, much more limited scope (like escaping from a boarding school that's more of a prison camp). And then it ends abruptly, because it's only 13 episodes, and that's pretty much it.
The series looks gorgeous, especially the backgrounds and the (digital) effects when a 'miracle' occurs. The mist receding very fast in the last episode was really something. The character designs are OK, if you're into that particular look. I would have liked a bit more realistic designs for this setting.
So, nice to look at, very interesting world, and plot arcs that are either very good or merely so-so. I'll give it a 7.5 -- could have been more with a bit more effort for tighter plotting.