I gave this a 9.9 on MangaUpdates. It's well-written and combines a depressive story with a bitter-sweet ending.
It's close enough to a masterpiece to count as one, yet there is something that keeps me from giving it the full 10 stars, not that I could name it. Maybe it's a bit too long, a bit too drawn out, short as it is.
But if you wanted me to name a specific example of something that could have been done better, I would say that would be the extra chapter at the end. Instead of adding it on as an extra chapter, I would have liked to have it immersed in the story itself, as it kind of gets in the way of the feels of the end.
My suggestion to you: Read the 16 chapters, then take a short break.
Once you've had time to digest the story, but before you entirely lose the sensations of reading it, come back and read the 16.5 extra. It's a good chapter, but it's also slightly disruptive where it currently is.
At the end of it all, only AFTER you have finished your read, check out the covers. They tell a little story of their own and will help you get back into the mood anytime, if you wish.