Before I say anything else, I should clarify that a 6/10 score doesn't mean this manga isn't worth reading. 6/10 is "fair" and that is exactly what this manga is at this point. Secondly, I feel like I really have to give two scores for Nisekoi:

For the first 45-50 chapters:
Genuine plot progression and character development, decent romance, and the feeling that the story was building upon itself. This part of Nisekoi is highly enjoyable and is the section currently being covered by anime Shaft is producing. It's a rom-com at its finest and gets a solid 8/10.

The problem starts after that....unless you like 60(sorry, did I say 60? I meant 160) chapters of filler. Gone is the character development, gone is the hilarious approach to love, in is an ever-growing harem of girls we know won't win and the abuse of gags. What starts as a truly entertaining love triangle blossoms into a nauseating dodecahedron with all the depth of a bath tub. It's like the manga is an airplane that has stalled and is now plummeting downwards, getting closer to a wreck by the week. With the latest girl added to the harem: the onee-san ninja chinese gangster who Raku happens to know, it is obvious the author is suffering from PSOOYA disease, a terrible conditions many writers suffer from. That is, "Pulling Shit Out Of Your Ass" My current score for chapters 55-current(118) is about 4/10, and that's being generous to say the least.

To understand why Nisekoi's plot goes off the rails and is replaced by bland moe-blobbery on a weekly basis, you have to understand two things. First; this is a manga that is published in Shonen jump, where long serialization is more important than development. Second: the demographic reading it doesn't give a shit. The popular vote for favorite character is very telling to the kind of people reading this in Japan, but I'll get to that later.

Story: 5/10
Starts off with an unoriginal but fun premise and is, for the first part at least, very well done. It would have a higher score if it had progressed an inch in 60 chapters.

Art: 8/10

While the art in Nisekoi can vary slightly, it always seems to get its point across perfectly. As much as I hate what has been drawn recently(moe,moe,moe!) it always does a very good job at representing characters and their emotions. No complaints here.

Characters: 5/10
"Five out of ten in a series based off its characters! Sacrilege!" is probably what the die hard fanboys are thinking right now, but with no development whatsoever in any character apart from Chitoge that is what they deserve. For starters, let's look at the MC.

Raku: Typical dense MC. Is at his most interesting/least cliche when dealing with Chitoge, but becomes a flat character when dealing with just about any other girl. Has a case of convenient amnesia.

Chitoge: First girl bias, incoming. A lot of people brand her as the "typical tsundere/bipolar" but her being cold to Raku initially shouldn't be surprising considering the premise. She hates Raku at the beginning but is forced to act all lovey-dovey when others are around, so of course she comes across as bipolar: that's the "joke". She's the only girl with some proper backstory and that has some level of depth to her character, but that's largely because she's the only one that went through non-contrived development.

Onodera: Fan-boys are going to flame the crap out of me for this, but she is your run of the mill cutesy girl character. A lot of people prop her up as "pure" but the truth is she's annoyingly indecisive and mind-numbingly boring. If you have a hard-on for shy girls and have never been in any sort of relationship in your life she's going to be your favorite character because that is her entire appeal. Surprise surprise, she's the most popular character in Japan.

Ruri: Best female character? Maybe. It seems side-characters are the only ones with heads on their shoulders in this series and in Ruri's case this is extremely apparent.

Shuu: Best male character? Definitely. He isn't dense and he's generally extremely fun to watch.

The rest of the harem: You'll fall in love with Marika if you stalkers who hold onto a love from when they were five years old and are ready to rip couples apart to get it back. "She deserves Raku the most" is a bit half-arsed when she doesn't know jack-shit about the current Raku and none of us know anything about her apart from the fact that she's pretty, talks like a redneck, and wants to marry Raku. She'd be a lot more likable if she'd been introduced slowly. Tsugumi plays her role well but her character development was contrived as it gets "he called me cute, character 180 time!". I'm not even going to get into the others, because we don't know enough about Chinese-teacher-don-nee-chan yet and I don't even want to get started on the little sister.

The 'fake love' between Chitoge and Raku is really the highlight of this manga.

Enjoyment: 6/10

For the same reasons as I put on "story", 8/10 for the first half and 6/10 for the second half, respectively.

Overall: 6/10

If within the next 20-30 chapters there is a sign that this manga is once more moving towards an ending it will go back up to a 7-8. At this point in time I don't recommend reading it because I'm not sure how the authors going to do it, but I can guarantee to you if you hear that Nisekoi got a "harem end" it's NOT worth the read.

EDIT:
After literally years it has ended. Years of filler. Honestly, give this thing a shot for the first few chapters and if you don't like the characters a LOT by the 50s or 60s, drop it. Not worth it otherwise. Ending was ok at least.