I'm probably just going to be repeating stuff that a lot of people have already mentioned, but that's because I feel the same way. This manga started out so good. I thought it was brilliant. The storyline isn't so much original, but the author's way of telling the story really brought the reader in, it was realistic, and it left the reader wanting more. Character motivation made sense, and like nekoichan said, the issues they were facing were believable.
And then it took a turn for the worse as the author introduced Narumi. It became heavily cliche, and the story telling got messier from there. There were chapters where things were advancing far too quick, it was so rushed like the author didn't think too much about it, throwing in all these regurgitated ideas one after the other, in the same chapter. Character was lost. Kou .. He feels very out of character right now (not all the time, of course, just every now and then). It doesn't even feel like character growth, it's plain OOC. I understand that Narumi plays an important role here for Kou to develop and see for himself how unhealthy her being in his life was. How he needs to move on. But as mentioned, there's WAY too much unnecessary drama. It's kind of suffocating.
I am severely disappointed, because I loved this manga so much. I looked forward to every update, I was on edge by the end of a chapter because 'no you can't stop it there!' It was one of my favourite shoujo romance's, which is why I am so upset with how this manga is coming to be. Also, I feel a lot of people agree with me on this, but I also don't want Kou and Futaba to be end game. While I do love Kou, a lot, his character really appeals to me (there is shit about him that I really do despise though, like, the fact he's kind of sexist. I know he probably means no harm, and that it's pure ignorance but still), I'm so weak against characters like Kou, but I want Futaba to end up with Touma. I feel this is best for all of them. The atmosphere of the story so far, makes me feel that if Kou and Futaba were to end up together, it'd be one emotionally toxic relationship. I don't know why I feel that way, but I do. That, and I admit I do want to see Kou suffer a bit more.
Of course, I am almost 100% sure that's not what we're gonna get. Touma, he feels like a tool used by the author to get Kou and Futaba together. His character is kind of really underdeveloped, and it doesn't seem the author has put too much thought into writing him either. So yeah. I don't feel this manga is going to regain it's brilliance any time soon.