The art is fine. Sometimes it's beautiful.
The main character is incredibly engaging. Some of the heroines are fleshed out well, some are one dimensional, all are run over by the plot.
The enjoyment is a roller coaster of emotions that feels like it's building up to an incredible payoff. Instead you get kicked right in the balls.
I can't believe it ended like that.
Slight spoilers ahead:
This is an omnibus story that goes through a bunch of relationships in order.
I don't understand why the girl the manga spent the longest time on acted the way she did once she started dating him. Her personality completely flipped. I can only weakly assume that she got bashful about realizing that they were dating so she stopped the playful banter and was too embarrassed to show any intimacy. But that's completely different from her character, unexplained, and doesn't really fit with the information we get after the fact. Although I can forgive a lot of that because the point of the manga is that you can't know why people act the way they do. It's still jarring to have that moral delivered in such a heavy handed way by dismantling a perfectly good character. Especially after his high school girlfriend had the same arc but shorter and actually justified.
The "mysterious girl" premise of the manga is a total red herring and an excuse to flashback though the guy's whole life story. Although that sounds like a clever and engaging story element, it has mediocre execution.
He does the same thing to the girl on the phone that he did to the girl at the Ramen shop even though she chewed him out so much about it. Although even that was just a variation of the same mistake the main character repeats every arc.
The main character drives himself into a corner repeatedly for over half his life in a deeply gripping, believable, and emotional way. This manga is so absolutely fantastic in this regard that I can forgive everything else about the characters and the plot. It doesn't hurt the portrayal of this deeply flawed MC that he continues to make almost exactly the same mistake over and over again. You've spent 7 volumes with this guy hoping to finally see him grow and be happy. He never changes from this self destructive behavior until the very last chapter, which is split into two parts and a hundred pages.
We get one panel of that (maybe) paying off.
I have never been so utterly disgusted by anything in my life.