I suppose a manga is really good if it makes different people hate one of the persons, but a different one to someone else, to the point they want to stop reading. I'm not sure if feelings of that much uncomfortable are a good thing for an artist to create, but it shows skill.
I disagree. The main problem I have with this story is that Adachi and Shimamura are clearly being made to behave in certain ways for dramatic effect, and not necessarily because it makes sense. Yes, Adachi was fascinated by Shimamura from the start, and it took quite a while to realize her own feelings. Shimamura, in turn, was always depicted as cautious, overly polite and somewhat baffled by the affection thrown her way by Adachi (and others). She never outright pushed Adachi away though, as she wondered about her own interest in that girl. It was subtle, sweet, sometimes heart-wrenching, but never dipped into straight-up melodrama.
Until now, that is. For some reason, the author deemed it fit to make have Adachi regress into a blubbering mess over Shimamura, and to turn Shimamura in a callous b*tch who just don't give a damn. None of this makes any sense within what was established about those two before, so what the heck happened?
Well, bad writing. That's what happened. My best guess is that the author simply has no clue how to prolong the subtle conflict and give any of the girls a convincing character arc. The "solution:" exaggerate the girls' personality traits the point of caricature. The result is a jarring mess that leaves me scratching my head, to say the least.