Before reading further, this is a review of the first 16 chapters of Kakkou no Iinazuke, the latest work of mangaka Miki Yoshikawa that got a serialization. Many of her works are shonenlike and with some cute moments, but this one is a full shoujo manga of her.

The story is a Romcom about two characters that were mixed up at birth, they don't know each other but are fated to meet anyways. There's the main character (Nagi) that discover after many years he's not part of his family, and goes to a reunion to know his real parents but it's interrupted on the road by Erika, which is the baby who got mixed with years ago.
Now comparing this story to others is hard, it's a fresh story and can remember other shojos like Lovely Complex to pull you through all of this freshness (instead of physical differences they're more of social status differences).
Perhaps because it's a serialization we'll meet many other characters that will improve the story, but at the moment looks like a romcom of one male character and many girls (his not biological sister, Erika, one studen at school, etc.) getting feelings for him. If it's not getting any male rivals on the upcoming chapters then the story's momentum will drop, and believe me comparing this to Miki's latest work (Yamada-kun to 7-nin no Majo) would be a failure of sorts.

Art is improved a bit from the One shot, by giving us a full page of Erika, Sachi or any other female character on cute clothes each chapter. The artstyle is simpler than in Yamada-kun but that's because the genres are somewhat different. you don't see many action sequences and somewhat because of that it looks stale, but I repeat, that has to do with the genre Miki Yoshikawa's working on.

Characters are solid creations, are well defined the tastes, love triangles and goals of each other. The only bad thing is that sometimes because the story is getting stale they have to bring out some stuff out of nowhere to keep it working. No even the interactions of Nagi and Erika will work on some chapters, and maybe Yoshikawa san needs to use new other combinations because if not then the ship will sink sooner or later.

Enjoyment is based on personal tastes, and until now I've liked Cuckoo's Fiancee, the good and bad moments it has, but I keep thinking that it's a 180º work series and needs some extra stuff in the future to keep it growing, maybe a part-time work for Erika and Nagi or a school event that will improve their relationships. The time will tell at the end how this will work.

Overall, if you're into Romcoms and needs some light content story, that will give you some laughs then Cuckoo's Fiancee is for you.
If you got a bad taste with how fugly was Nisekoi then maybe this is not for you.
And of course, if you liked the manga and anime adaptation of Yamada-kun to 7-nin no Majo and the works of Miki Yoshikawa, then of course this manga is for you.