I personally like the way things turned out. Shizuku confesses to Kaori and realizes that her love was requited, but isn't able to actually go see her- when you lose someone close to you at such a young age, it's always too soon. It's also sad that Ruri and Seri didn't get to see Kaori one last time.
Speaking of their relationship, I also liked the progression of it. Shizuku starts off indifferent to/annoyed by Kaori, but you can see signs that Kaori is growing on her, some more obvious than others(for example, in Chapter 5, she doesn't care whether Kaori uses her first name, but in Chapter 11, she actually asks her to do it), and becomes more of an active participant in the relationship over time.
I also like the pacing, since while some chapters are more important than others, something significant happens in almost every chapter. The fact that there's a fixed duration for the story helps you tell how close it is to the end, and the fact that it's relatively short means that there aren't all that many parts when nothing of note is happening.
I'm looking forward to the final chapter, along with the Volume 6 extras, and personally hope that one of the extras reveals what happened to Kaori and Shiori's mother. She's implied to be dead, since Kaori says in Chapter 16 that she knows what it's like to lose someone, but nothing explicit is said about what happened to her.