The premise, the general start/mystery, and the art were good, but it's everything in between that could have just been snappier. There was a whole lot of nothing/filler for the better part of this story...I got bored and just skimmed from chapter 20 to the end pretty much.
As far as gender benders go, it's as realistic as it's going to get and that's why the beginning was so interesting. However, the plot isn't really tight and things aren't well explained. This author usually likes vagueness and a psychologically messed up cast, but this story left too much to the imagination when it had 80 chapters to work with. It's not so much things need to be spelled out, but things flat-out just weren't clear. For instance, the whole 10 or so chapters with Mari's original friend group. It's clear these are shallow, hollow friendships and I suppose the author is trying to say that "real" Mari is a shallow, hollow vessel, but...okay? What's the point though? Did we need to spend so much time with these characters just for it to go no where? The love interest in this is also a piece of work and I didn't find their romance remotely endearing. They're teenage girls so I forgave the mellow-drama, but as someone said, it gets day-time soap-opera with it. The love interest also has their own life drama and that's hinted at and resolved, but never really explored in any level of depth.
Above all things, the "twist" was...anti-climactic as hell, to say the least, and beyond dumb for all the build up. I thought something horrific happened, but in reality, it wasn't that bad and because of that, I wish we'd learned more about Mari's parents relationship beyond what's hinted at and how & why it is that way since that seems like a HUGE part of why Mari experiences life the way she does. I also wish we'd gotten more flashbacks of the dynamic between Mari and Isao because we get some major things dropped on us in the last five chapters. You know those old detective stories where, during the final scene, the detective reveals THE mystery-solving clue and it's one that wasn't shown to the reader the whole story until that very moment, meaning the reader was left in the dark until the last moment? That's basically what happened. And then after allll that drama, everything wraps up WAY too neatly with a time skip. No talking, no addressing Mari's behavior, nothing. Just skip a year ahead.
My frustration with this manga in general is all the hinting without showing and the characters being half developed and all over the place. The pacing, as far as the story goes, isn't bad -- the author doesn't take you on a wild goose-chase or pull out a bunch of annoying red herrings, but it's slow and a bit too meandering slice of life for a mystery like this. Oshimi always adds a 'perverse' element to this work, which I appreciate, but here it was distracting and it could have been incorporated earlier. I really want to recommend you read it, but there are too many elements that are just half-baked from my perspective. Read if you are a fan Oshimi's work in general, but if this is your intro to this author, it's likely not the best.