I find this to be extremely sweet, and rather brilliant.
The author introduces a world where gender is a naturally (in world) changeable thing, where your gender can switch based on a balance need. This happens now (as in OUR world now) in creatures like fish, so it isn't totally out there.
In such a world a change would be understood by all, and if it isn't a "usual" thing that happens to everyone, it isn't an "unusual" or unknown thing, either. No way, in other words, to play off a person's gender change as anything other than what it is.
So, when Mamiya becomes a girl, there's shock at her changing when she is so old (it is more common to change as a child). (It's also mentioned that the older you are, the harder the change can be, thus leading to a higher mortality rate, and if you know anyone who is trans, or just know statistics of that group, suicide among people whose gender perception goes against the social norm is very high, so this is a nice little toss in for real-world relation.) They also accept her as her quite quickly. As he makes an attempt to do the same. It's not easy (can you IMAGINE having your first period as an older high school student when you had no idea it would come, but as a guy had some vague understanding ... and probably thought of it as gross ... but no real knowledge...), but as a character who has had unexpected changes in his life (his mother leaving when he was a child, for example), he has a mental foundation to begin accepting his new self. As well as an ... if not entirely supportive (he's afraid of women!) friend, at least one who cares for her regardless of her gender. Though the care changes a bit after boobs...because hormones, obviously.
I've thought about it a lot...if my best friend when I was that age became a guy, I would have fallen in love with him, no question. (I was more than half in love with her in the first place, lol) So. Maybe that's why I found this manga so much fun. It's got a lotta cliche's, but that twist of freshness, friendship, and self-discovery make it worth the read.
Also, it's only 5 chapters. It's not all that great an investment in time! Read it!!!