Contrary to many popular beliefs, I don't think the ending of 5-Toubun no Hanayome was bad. The people complaining make me think that they are only mad because their favorite girl didn't win, which isn't fair. Even I was rooting for someone else, but I guess such is life.
This manga doesn't do anything new, even I who has never read a harem manga before can understand that, but for a work to be successful it's not necessary to be a path-breaking product, you just need to make us care for the characters, and I believe the Mangaka was successful in doing so. The series is named so for the Quintuplets, and each one of them was atleast an interesting character, although only 2 got good character development throughout the series, which is a bummer. The main character also was good enough, who had his own motives and quirks, however silly, and wasn't a bland cardboard harem protagonist. He actively goes out of his way to care for the sisters, and you can tell that he isn't in it just to smash. It is fun to see the relationship between MC and the sisters grow throughout the series for the most part, and if you're looking for something light and enjoyable to read, Go-toubun no Hanayome won't be the worst thing to chose.
Now onto some spoiler-ridden rant:
As I said, I was satisfied with character development of only 2 sisters, and they're Nino and Miku. You hate Nino at the beginning, but she goes through a good-enough growth arc to become an entertaining character. Miku was always good, but she works on herself and gets more confident throughout the series. Seeing her grow was heart-warming.
Now onto the girl that I was actually rooting for, Itsuki, she started out as a likeable character at the beginning, but stops being of any relevance to the plot at one point. Her arc was the worst of all. I mean, I'm happy that she goes on to follow her dream, but the Mangaka could have handled that in some more interesting way. Instead we get something that feels like filler. If her arc was done in a better way, I'd have given this series an 8 or 9. Perhaps that is my bias.
If you have interesting characters, you're bound to have bland/un-likeable ones too. Ichika was the one I was least interested in seeing, as there was never a clear direction as to what the Mangaka wanted to do with her. Sometimes she is the caring big sister, sometimes the cunning brat, and sometimes the shy tsundere. I honestly wouldn't have cared if she wasn't there in the first place. And then we have Yotsuba. She might be the least popular one, and I can see why. Jonathon Joestar is also the least popular Jojo. It's because people don't like bland do-gooders. They prefer tsunderes and cute waifu-material girls in harem, and Yotsuba was a generic good girl character in the story. I understand if you end up not liking her, but I place her above Ichika as I don't mind characters like her. She has spine, and can stand up for herself and make her own choices. In the later chapters we get some chapters from her perspective which was cool.
I also like the character of the sisters' dad. It's a shame the dynamic of MC and Quintuplet's fathers wasn't developed further and stayed practically the same. You'd also see the mysterious girl "Remi" appear throughout the series, and although the way it was handled wasn't perfect, it could have been much worse.
IMO the series was at its peak when Futarou was teaching the sisters for the tests, and then that aspect got lost in favor of high school festivals, which is a shame. there are too many festivals in anime world for me to care about. Every high school series supposedly has one. The Mangaka ditched something which had a touch of uniqueness in favor of generic stuff, which was a bad direction for the Manga.
Well that's it from my side. Thanks for reading