just got done watching this series and I only have 2 words to describe it, "simply beautiful."
The story itself is really good. All the characters get really well developed throughout the show and by the end you just love them all ha! The mood changes in Sora no woto are perfect, starting off light and funny, then moving toward the more serious as the series nears the end. Although the only thing that I didnt like is that some elements of the didnt get fully explained. There is a 13th episode but its more like an ova and doesnt really deal with the main story of the series.
The animation in Sora no Woto is absolutely stunning, by far the best I have seen. The background, lighting, and character designs are simply breath taking. Its just pure enjoyment for the eyes! It definitely has a more realistic drawing style to it. So the characters dont make crazy faces or doing anything like that. Which really helps to set the mood of the story.