The whole show has such great atmosphere. My only negatives with the entire show were the character designs for the girls, which were pretty good other than the faces. I just think they were a little too square-ish. I loved the music. The music is one of the things that actually makes the series so great for me. I actually listen to the soundtrack independently from the show all the time.
Past that, without spoiling anything, I will say that the ending is very satisfying and my only other major downside is that they didn't really show much about Sayaka, just her boy's (forgot his name). Even past that, her characterization is great.
Anyways, this is my new favorite of all time. It's just so surprising. I wasn't expecting anything going into it.
