It sucks - 9/10
This is a perfect review of the series.
Manga is pretty much on point until it passes roughly ~80% of work. Most of the read is extremely enjoyable, fun, exciting, intense, erotic, feel-good experience that I had pleasure of familiarizing myself with for (almost) three days, before the work started showing signs of injury on better part of last day. Crazy adventures, situations, troubles, laughs and disasters our cast goes through in their lives is pretty much the definition of youth everyone would like to experience, maybe, to their own risk. Then, sadly, two things happen.
1) Prolongation
2) Bad ending
First, it draws out, for a while, then, as many already said, worst ending happens, essentially enraging everyone and justly so. Why? Because it's not a proper ending, you can't even call it one. To be frank, it's so silly (in a bad way) I'm not even taking it as a genuine conclusion. It's just not believable whatsoever and it should be disregarded as such. That is not to say I'm attempting to delude myself into not accepting Akira Hiramoto's decision-making. It's there, yes, yet it can always be altered, buuuuuuuuuuut, that's the writer's duty, not mine.
Anyway, the ending itself is completely irrelevant in my eyes and I'd like to mention that if there was a reconstruction of aforementioned remaining 20% (to the better, of course), more precisely when the story started dragging, it would be 11/10 in a heart beat, that's how amazing the manga was. I shall mention several concise spoilers at the bottom of this text for few who wish to understand exactly why people dislike it so much without indulging into reading it all.
Now, I shall briefly display all volumes' ratings:
arc 1 - 10/10 - vol 1. - vol 9. <-- anime adaptation(ch. 1-88)
arc 2 - 10/10 - vol 10.- vol 17.<-- a 'would be' s2 (ch. 89-166)
arc 3 - 10/10 - vol 18.- vol.22.<-- best part of manga (ch. 167-218)
arc 3 - 6/10 - vol 23. & vol24.<-- starts dragging (ch. 219-238)
arc 3 - 7/10 - vol 25. <-- a false glimpse of hope (ch. 239-248)
arc 4 - 6/10 - vol 26. & vol27.<-- full standstill (ch. 249-268)
arc 4 - 1/10 - vol 28. <-- a complete catastrophe (ch. 269-277)
(22*10)+(6*2)+7+(6*2)+1/28=9.0 = 9/10
So, overall, we can conclude it's a great series, though, when ending is included, it ruins all work invested, and severely cripples established impression and rightly so.
Lastly, I shall now mention what I was dying to mention this whole time, and my overall perception on the whole thing. Those of you who do not wish to be spoiled and wish to read/continue reading the manga, which I still firmly believe it's worthy of such an action, please cease to feast your eyes on this God-given gift of content, and derail your attention elsewhere.
!CAUTION!WARNING! Spoilers ahead !CAUTION!WARNING!
Basically, while digesting from vol. 26 ahead there were so many errors it's not even worth hairsplitting every detail. I'll just mention 'no-no' moments which pop into my mind as I'm writing.
Mari vanishes without explanation and she's essentially never touched upon again, even though she's one of the main characters. Mari mentions she's dropping from school around end. No explanation. She was my favorite character and for some of you as well. Let that sink in.
None of the boys or girls are confirmed to end up together, it's simply left vague and unclear. Only pair that is successful out of all people is with Risa and that disgusting slob Andre.
No epic last scene wet t-shirt event or no full cast happily hanging around some sort of barbeque, beach or whatever.
Oh and the whole Chairman arc was unnecessary filler, if you just had him appear towards the climax of cavalry battle, it would take away nothing from the experience.
Most importantly, who does Kiyoshi end up with? It's what everyone wonders, right? When it comes to Mari, it's just discontinued like it never existed. Kiyoshi rejects Hana at very end and successfully confesses love to and is accepted by Chiyo but in such a repulsive, rushed and forced fashion it feels crappy, like a punch to the gut. To be honest, it was clear whoever Kiyoshi ends up with wasn't gonna satisfy everyone because the writer himself couldn't decide, nor care. Obviously, he would end up with one of them and whoever shipped one of the other two would be left, at least, pouting at the result, which is an okay, acceptable solution. Hell, I'd even be okay with harem ending. But even after that, KiyoshixChiyo also failed, because, well, because Kiyoshi wore Hana's panties, and that was too much for Chiyo. And, then, Chiyo becomes man-hater - the End. This... This, this was just sacrilege. After delaying this finale for two volumes just to not deliver it properly at all, it's better to scrap the whole thing and wish for a better end. Manga felt like it needed two-three more volumes at the very least though preferably, say, five volumes and a better, not forced, but smooth and satisfying finish which it failed to deliver.
tl;dr
kiyoshi x mari - no ending
kiyoshi x hana - no, at last moment (awfully unveiled)
kiyoshi x chiyo - yes, at last moment (awfully unveiled), and then no, wtf
gakuto x mitsuko - no ending
shingo x anzu - no ending
joe x satou - no ending
andre x risa - yay, who cares