Its has a mature story between 2 people. The dynamics of the relationship between two girls. It was good. I am glad that there are lots of people in anime doing this kind of thing than far easier crowd pleaser kind of stories. Implementation/subtlety/story/psychology was not perfect/surprising/amazing, but it still left we thinking and appreciating friendships and the need for empathy and how easily we can misunderstand ours and others feelings.
Not for all. Not even the regular sound euphonium viewers. You could skip it and not loose the story line. Mature people might enjoy it more.