I actually laughed during the first two, maybe even three episodes. Despite the clear dumbness and exaggeration, part of the comedy hit the mark. The protagonist's arrival to another time and initial interaction with the male lead were good.

But My Only Love Song runs out of that very quickly: after those episodes, the vulgar side of the comedy and particularly the absurdly frequent and long repetitions (sometimes a third of an episode's running time!) stuck out too much to continue. With the new characters' arrival (eps. 5-6) it no longer made even simple sense.
I would've dropped it there, but I didn't due to funny circumstances that lead to going over the first half and thus wanting to get it finished. At first that choice seemed good enough, because with the protagonists' relationship becoming clear the drama turned a bit entertaining again, and it was possible to see all the foolishness around them and the "villain" as something amusing instead of stupid.

Yet the improvement didn't last long this time either.
From there, the series switched to drama and wasn't better written any for it—quite the opposite. That it tried to cope with more serious events showed how weak the writing's quality is. The plot and the pace were always messy, but I was especially disappointed in the characters' writing turning extremely unsteady and incoherent. If in one scene a character clearly feels despicable and in the next one you kind of like him, and then he's treated by the plot as despicable again, this just shows great inconsistency.
With how little I've watched, there were little gestures I still thought were specific to certain characters from other dramas, and My Only Love Song pointed them out simply as K-drama clichés, so it was also the cause for some disenchantment.

There were things I liked at the beginning, and I was curious about how the story would fare out. At some point I also thought the characters were peculiar enough to follow, until they also progressed insufficiently. Both the initial ideas and their later execution showed more holes as the episodes went on. Even simple aspects like the music were mixed and applied chaotically, not to say with very little taste.
It also didn't help to find out about Lee Jong Hyun's misconduct mid-way, as that added even more conflict and unpleasantness to watching it.

Overall, with the exception of the initial and some middle episodes, it worsened progressively and, despite being hesitant along the way, I finished it feeling clearly that My Only Love Song wasn't worth picking up, and even less finishing.