I have to say, I started watching the drama (mainly because I recently discovered Lee Do Hyun) and I really liked the way it started. However, then from episode 14 (or maybe even 13), it took a turn in the wrong direction.
But let's do this neatly.
First of all, I liked the ML. In my opinion, he is a very good actor who could convey his character's emotions, habits, etc. quite convincingly. Sun Gyeol, his character, is not someone to be envied in this drama. He is a poor soul, suffering from childhood trauma, abuse from his tyrant grandfather, and he mother who was absent most of the time (although I liked her development).
Kim You Jung did a pretty good job as well, although she seems too young for her role. I liked her character at first, Gil O-Sol seemed to be a pretty strong young woman who didn't let failure come in her way. However, in course of this drama O-Sol turned into someone who could not speak one sensible sentence... quite annoying.
Song Jae Rim provides a strong case of second lead syndrome... I loved his character and I wanted him to end up with someone who deserves him so bad... but his "end" is so frustrating. He deserves so much better. I would really like to see him in a main role where he gets the girl.

I loved the side characters! But here is the thing, they had so so so much potential which wasn't used at all! Gil O-Dol aka Lee Do Hyun needs to have more screen time, especially more scenes with Joo Yun, they had so much chemistry and such a sweet story going on. The cleaning fairies are so funny, but also have interesting background stories, so we definitely need to know more about them!


Finally, spoiler alert:
The story develops quite ok, you have some typical k-drama tropes, but then, as I have hinted before, the story takes a dark turn. I really don't understand why they needed to break up Sun Gyeol and O-sol. And why she wasn't able to tell him why.... Sun Gyeol had absolutely not idea what was going on, but everybody left him in the dark! So frustrating and bad storytelling, I have to say. And he was completely innocent and had nothing to do with the whole tragic incident, but was made responsible. I didn't understand his grandfather as well... what exactly did he try to achieve? And why was is so easy for him to be redeemed??? This storyline wasn't believable at all. Additionally, the way Sun Gyeol was treated by O-sol and her father when he came back was sooo annoying. He didn't deserve such a rude treatment at all.

So all in all I cannot really recommend it. I watched this drama mainly because of Lee Do Hyun, I think he is amazing actor, so I don't regret the time I wasted watching, but I think there are way better romcoms out there which have better plots and more interesting characters.