It was pretty good. I thought the twist was reaching a bit--like, to pull it off they had to kind of stretch the credibility of the first part. I think it's actually hard to do a good unreliable narrator in manga. I mean, to me the thing that makes unreliable narrators work is that they have an implied audience that they have some kind of awareness of--they're telling the story, and they colour it in some way to fit what they want you to think. In manga, you've got a viewpoint character, but normally they're not telling you the story--they're unaware of the fourth wall. And so if a character's thoughts turn out to have misrepresented how things are, particularly if they misrepresent how the character is perfectly well aware things are (ie it's not that the character has delusions), to me that comes off just implausible, or borderline cheating, or at best just kind of clunky.
This twist was pulled off mostly by having the character not think about certain aspects of the interactions until it's time for the reveal, and it pretty much works . . . but it's a bit clunky to me, because frankly it would be really weird for someone's stream of consciousness recollecting events to be structured in that way, where thoughts about certain aspects of their relationship get delayed even though clearly as events happened they were mixed in.
So I'm not as wildly impressed as many here, but I still liked it.