A word of advice to people considering whether or not to read this:
I would strongly recommend watching the anime instead.
This series is a fantastic comedy. It's hilarious, it's cute, it's a magnificent niche type comedy. The setting is unique, the cast is funny and entertaining, and the protagonist is relatable.
That said, it is a terrible, terrible romance. Being a (reverse)harem, all, and I mean ALL of the character development in any of the supporting cast revolves entirely around the main character, Haruhi. Of the six male leads, four of them are romantically interested in the main character, and it's their conflicts about her that drive the plot. This type of storytelling is not a problem, but this series fails to capture the charm that other good harem works have not only because the writing is pathetic, but also because the author pretends that the growth of the characters is not caused by the female lead. The story plays much of the development off as being internal conflict in the characters, or conflict between different members of the Host Club, but each and every conflict can be traced back to any character's interest in the female lead. This is just lazy writing.
Worse, even, most of the "character development" that happens is reversed as soon as an arc finishes. At the beginning, the personalities of the characters are pretty interesting. They are fun to read about, but have room to develop as people. Unfortunately, they don't. Each character has a distinct "personality", an archetype they portray as members of the Host Club (think acting club, but more flirting). In order to preserve the comedy of the different characters' personalities clashing, the author makes no changes to their personality at all. Ever. Of the seven characters, I can confidently say that three of them had no growth and the other four had slight personality tweaks. Once again, this is just lazy writing that is supposed to convince you that the plot is driven by character development.
The cherry on the top is that the characters are just annoying. Some shows are able to run purely on character interactions. Be it comedy, fantasy, tragedy, drama, whatever it is, many genres have works that are carried entirely by interesting characters and fun dialogue. Ouran High School Host Club is not one of them. The characters are flat and boring, and any attempt at making them more interesting characters comes from the single conflict that they have over and over again. The poor main character, Haruhi, is constantly annoyed by how rich and oblivious her classmates are. The drama queen male lead, Tamaki, has daddy issues. The serious, conniving character, Kyoya, doesn't know how to function like an emotionally stable human being. The incesty twins, Thing One and Thing Two, have self identity problems because they constantly act as a two-part twin act. The shota-but-he's-actually-18 charater, Honey, and his stioc bodyguard, Mori, had problems so insignificant that I can't remember what they even were. The author makes no attempt at writing interesting struggles for the characters, and even the finale is just Tamaki's daddy issues arc #37.
These problems with writing interesting stories and romances aren't just an Ouran thing. The author's other works are like this too. I forced myself to read them so you didn't have to. Don't do it. They're painfully bad. I honestly don't know how they have ratings above a 3.
The anime, on the other hand, focuses mostly on comedy. Things get weird after the beach episode, but it handles the comedic episodes well and understands what makes the manga readable and not utter garbage. The writers understood what made the characters funny (wOW), and they made the effort to make the character conflicts funny and interesting at the same time.
TL;DR: The manga is tedious and boring, and the anime took all of the parts worth reading and made it better. Please, go watch the anime.