Okay, I'll bite.
! The premise is a mixed bag. It's good to have the author introduce these promises and keys and whatnot early on, because nothing would be a bigger copout to have them validate some relationship late into the manga, as if to suggest "you were the one all along". At the same time, he's played his cards early - you have the girls in play, so now it's just slow build up until something happens. Which, if the works of Rumiko Takahashi are anything to go by, that is extremely annoying.
! And yet…..it's like each chapter pulls Raku in the direction of one girl, then goes to another...I have no idea what the end-game pairing could even be. Maybe the author doesn't either, and that could be dangerous. But I do feel as if there are emotionally developing moments.
! At the same time, it feels like a harem angle is there, like the elephant in the room, but….not really viable? It helps that unlike a lot of harem series these days, there's no supernatural girls or alien girls or monster girls or something like that, it's a little more grounded. I say "little" because you still have the feuding mafia family angle.
! And that's partially why I feel a harem couldn't work - as per the double standard of our day, Raku's associates aren't too bothered by the idea of him having multiple girlfriends, but Chitoge's group certainly are. It kinda defeats the purpose of the whole "pretend date to make peace!" idea on both fronts.
! ...Which brings me back to the love interests. I (mostly) do like them all, mostly I'm tethered to Onedora and Chitoge, and both have their pros and cons.
! -Onedora is the sweet girl next door type, and she likes Raku as much as he likes him. She's clumsy and kind, of course the two of them struggling to realize they like each other is as frustrating as it is endearing. (on that front, Ruri becomes like my favorite character, she's awesome). Such a vanilla pairing would be fulfilling, but a tad cliche.
! -Chitoge is more firey, more blunt, and she and Raku start off hating each other. Naturally they come to learn more about each other, become more civil, slowly gain feelings for the other....suddenly the idea of them "pretend dating" becomes less out there and more genuine, even though they'll never admit it. Or will they? "Becoming the mask" is the trope of the day here, and like Onedora, the situation is fulfilling but at the same time, who didn't see this coming?
! So I feel they both polarize each other, but darn it all, I still like both set-ups.
! Here's another way why a harem route can't really work…..normally the girls are aware of each other's feelings and support each other (like urgh, To-Love-Ru or something), which isn't super realistic. Obviously Chitoge will not admit she likes Raku, partially because she herself hasn't realized it, but she also isn't aware the boy Onedora fancies is Raku, although Onedora becomes aware that Chitoge has to pretend date him.
! They are amicable, but will tensions shift? Who knows~
! Then we move down to the other choices
! Marika - she's amusing, I like her pish-posh fancy attitude covering up a less refined valley-esque girl. Plus her dad is chief of police, ha. But I do feel throwing her key into the mix was contrived, even though at some point, the keys and the lock will not matter.
! Raku is a teenager, and at some point, real-life feelings become a lot more important than a glorified promise when you were young.
! So I feel she's a bit of a late comer, plus this is the world of shonen: you show more active affection towards, the less chance you actually have. But hey, I'm only on volume 7, let her story be told a little more.
! Tsugumi - woof. Tsugumi is up there with Ruri to be my favorite character, but I really don't like her as love interest choice. When she got introduced as this (male) cold assassin type, naturally I wasn't expecting the story to pull a 180 and have her develop feelings. But that wouldn't be so bad, if the fact that she's actually a female (gasp!!!1!) didn't get hammered into us by the author. Yes, we get it, Naoshi Komi, you stuck a bow on her and had to give her the largest bust size. Plus showing off her figure in skimpy clothing via Chitoge plus a very questionable scene of her teacher, her own breasts, typical shonen girl bath fanservice, feeling up…..urgh, the line was crossed a little.
! ....I don't know, things weren't handled so well (oh my god, did I just make an unintentional pun)
! You can't have your cake and eat it too, so while seeing Tsugumi struggle with her own feelings (and the fact that the boy she may like is "dating" her mistress) is interesting and worth investing in, I really thought having her have a forbidden crush on Chitoge would be more interesting and organic (if just as cliche)
! Shu is your typical perverted best friend, but hey, he has his moments of wisdom. I really liked this one scene where he tells Ruri to let Raku and Onedora figure it out for themselves, despite her efforts to get her best friend with her clueless crush.
! There's other good moments, like the chapters where we get introduced to Chitoge's mother, Raku trying (and succeeding) to get them to spend time together…..
So yeah, Nisekoi is ridden with cliches but it's a guilty pleasure for me, as the wall of text can varify. Forgive me, Arlong Park.