First of all I won't analyse the quality of the manga itself, that's why I refrained from rating it. I do have to say though, I dislike it but that's not very important, I just want to make a point since I didn't dislike it because I thought it was bad but because I immediately noticed it wasn't within the genres I like (shounen, seinen).
Let me start this by saying that I'm not criticizing the series itself, just trying to make a point. Now let's face it: Horimiya has got to be a Shoujo manga. Seriously think about it. Imagine you hadn't seen what genre it belonged to and were asked to guess it. I'm guessing that if we polled that and people answered completely unbiased (without having seen the genre before hand) the answers would be: 70% shoujo; 25% josei; 5% shounen/seinen/"no idea". If you're gonna say "but it comes out in a shounen magazine" I'll accept that but also point out that said magazine contains a shounen ai manga (Utau! Heiankyou), a shoujo manga (Tokyo Yamanote Boys) and 4 manga that eventually got transferred to shoujo magazines (Gestalt, Hatenkou Yuugi, Switch, Tactics) so it wouldn't be weird at all for Horimiya to actually belong more to the Shoujo label than Shounen.
Now you might be saying "yeah, yeah, whatever douchebag what does it matter?". Well, it matters quite a bit. Let me state this very clearly first: I do not think and am not saying that only women read and enjoy shoujo, or josei for that matter. Shoujo is just a sort of marketing label which indicates that the manga was mainly targeted towards a female audience, it doesn't mean that its readership or fan-base consist exclusively of women. Just as there are many many women out there who really enjoy Shounen or Seinen, I believe there is also a good amount of guys who appreciate and even prefer the genre. The thing is, we all have preferences. And a good amount of us have some genres that they just don't like. For instance, I do not enjoy shoujo or josei. I've tried a few before and quickly noticed that it really did nothing for me. Not because they're bad, but because it's a genre that really doesn't appeal to my taste in particular. That's all. And that's why these labels that fundamentally are nothing more than marketing labels end up being important when I'm looking for new manga to read. I mean, just think about it, it narrows down my search results to nearly half.
With that in mind, allow me to make a rather bold and particularly risky guess... I'm guessing that most people who really love (and I mean it, not just kind of liking it) this series are women. I apologize to any male fans for this rude guess, but I think that it helps my point. Horimiya is a shoujo series. I don't see how it isn't. If you don't think so I beg you to please try one thing: think back and remember the first chapter and think if it'd be weird if that was a shoujo series. Maybe not the classic absolutely generic shoujo but still a shoujo. No 2 ways around it, the story is perfectly shoujo-like.
That being said, to any out there who, like me, have tried the shoujo genre only to find that it did not suit their preferences, I can not recommend this as it absolutely feels like a shoujo story in every way.
I apologize if I have offended any fans out there, it was not my intention, I just wanted to state this for others like me.
EDIT: Oh and trust me, I'm not saying that just because this is a romance. I'm not that new to manga.