Since this is an ongoing manga I'll treat it as such while not discounting what's happening with the latest chapter (29 if you buy the manga, 27 at translation)
This is a harem romcom, plot doesn't matter. What counts in this genre is how many cute moments can you cram in a tankobon (or how many lewd ones if you're Maken-Ki).
On reviews for this manga on Amazon JP, some I read rightly pointed out to the selling point in this series: Gap moe. Three of the four main characters certainly satisfy that description.
There is exactly only one serious love interest though. You have to be shitposting on 4chan to deny this.
The girls are all cute, but they have uneven screen times because of the above statement. In essence you can drill these archetypes to (CV's my own)
I love the characters for what they are. I unabashedly like cute girls and these girls are pretty cute. The cute art only makes everyone more loveable and our MC more handsome.
Overall I shrug cliche's and bland stories off. I don't read romcoms for interesting characters or subplots (big plus though), and the girls are cute enough to keep me going.
Talking about plot, below is a detailed rant / review over chapters 19 - 28.
I am going to assume you have read to chapter 27 at least.
The author tried to stir drama and fucked up. NTR in a fucking harem anime? I'd rather go watch School Days holy shit. The even bigger problem is how rage-inducing this is if you bought into Adagaki as a character.
Almost completely destroying a character and halting any progress for 7 chapters is not entertaining. Having a fat lookalike of Masamune-kun in the first place is beyond fucking stupid. As a writer, you usually go for the more entertaining, the more interesting route in plot development. As of chapter 29 I see absolutely no idea why we even need this character and why write him in such a way.
Double-kun is complete filler. He has no significance other than to tell us how Adagaki felt about MC in the past, and her relationship with him, which could easily be done without him.
For example, everyone hangs out at Adagaki's house because Shenanigans, and Adagaki and Koiwai are stuck in a closet outside where Adagaki talks of memories of Piggy to Koiwai because she thinks no one knows anyway or that Koiwai didn't interact with Aragaki before Piggy left. Koiwai decides to keep this a secret. This gives more exposition to both Aragaki and Koiwai
The only meaningful contribution this method had was the almost artificial feeling of "I can only really love the true Piggy", as if her suddenly going dere in 1 fucking chapter doesn't negate that whole exercise.
Consider the fact that if Adagaki had no trauma over Piggy leaving her, she would just unquestioningly spread her legs for Double-kun and MC hangs himself for losing the girl of his (subconscious) dreams to the one thing that invalidates half of his existence.
We are shown nothing to hint that Adagaki would choose MC over Double-kun other than Double-kun saying he doesn't see Adagaki's past as important, which she disagrees with strongly. In that scene, her past is revealed to have a very important bearing in her sense of self. In the whole manga, only that scene was a source of conflict between Double-kun and Adagaki.
In an alternate world where Adagaki doesn't particularly care why Piggy left him, just that she always liked him, the pure-hearted Adagaki will, as per harem manga character trope rules, devote herself solely to Double-kun and shrug off oft-forgotten childhood memories like all normal people do.
For me the chapters 19 (last few pages) to 28 did not exist, but I omnisciently know how Adagaki feels. This is how I reconcile the bad parts and still like the manga.
By chapter 29, I came out feeling like I turned to the next page of a novel to find that our dear heroine has been hit by an airplane and recovered from amnesia overnight.
The bigger problem in terms of character is that the rival is not endearing as a character. The story tells us he has made a lot of friends in a day, but we don't see why (his friendliness or character), nor do we see any evidence of him being a genuinely good guy pressed in a hard situation financially. He's framed to be an opportunist who's NTR'd MC, which just grates on you.
How did this development destroy Adagaki's character?
Adagaki in her monologues confesses that she hurts others because she is afraid of being hurt. More than likely, her bashful nature translated into a negative image people had of her, and she had no choice but to live up to it (as is standard manga fare). This goes on and she's now stuck in an environment where everyone expects her to be cold, and she can't show her true self but also doesn't want to be hated.
For some inexplicable reason, suddenly Adagaki is okay with opening up to someone she abused relentlessly when she was a child, basically negating her whole thought process without given due reason or komas in the manga. And this after not meeting the guy for 8 fucking years!! That is mind-numbingly stupid.
If there's one thing Nisekoi does well, it's write endearing filler chapters respecting and at times moving forward established characters, while still being entertaining enough that people won't mind reading it.
Rant about writing quality in the following spoilers are based on chapter 28 of the manga which is untranslated as of time of writing.
We find out in chapter 28 that Fat Double-kun is a girl, just like Tsugumi in Nisekoi ffs.
This is one of the worst variations of "Rival appears!? Or not!" tropes in manga, because the character is both annoying and destroying our main heroine's character.
The reader is led on a ride to believe something the author knew was simply dishonesty as shock value which is absolutely lazy writing. In middle-school narratives, this is the "It was all just a dream" trope translated into harem form.
The whole story was building up towards MC having a real annoying rival, but then POOF! He's a fucking girl you twat!! This pang of annoyance boils down to: "Is that it? You led us on to expect some shit to go down and he's magically a non-factor?"
This is why the "It was all a dream" trope is so discouraged. It's anti-climactic. Fat Double-kun did tell the Yuri Brigade that he doesn't wish to get close to Adagaki at all, but how are we to believe he didn't have an ulterior motive when pure-pure Ojou-sama did? That scene did nothing but show that Double-kun bought the trust of completely-irrelevant-except-for-this-arc Yuri Brigade to be reduced to comical henchmen (kendo girl was cute though 8/10).
Writers who purposely set out to deceive with such obvious dishonesty tread a fine line between an annoying lie and a lie that is entertaining enough to be shrugged off.
This is different to writers introducing completely new characters from the blue (Ojou character) which is a bit jarring. A similar anticlimactic development would be if the authors were going to invalidate her whole existence by telling us she was hypnotized by Masamune's dad to like him.
Either way, nice manga. 10/10.