The reviews and many of the comments for this drama seem to be from people who didn't really get what this drama was trying to do and don't personally enjoy dramas that move at a slower pace, which is rather unfortunate as that has unfairly colored their reviews, in my opinion. Yes, this is a slow drama, but that is intentional. You need only look at the title to be able to see this.

Andante- moderately slow; usually used as a direction in music

It's perfectly understandable that slow paced dramas are not for everyone, and they're honestly not for me most of the time either. Even with Andante, a drama I clearly very much enjoyed based on my rating, I took multiple long breaks in-between watching episodes, because it really was just slower than what I enjoy in a drama. But that is not a flaw in the drama. That is a personal preference that I fully recognize. If you do not enjoy slow paced dramas, I recommend that you maybe watch something else, because you might not be able to fully appreciate the story this drama tells.

That being said, if you enjoy slow paced dramas with simple but affecting storylines, then I highly recommend Andante. It's thoughtful and moving with stellar performances by the entire cast. Even Kai does a decent job. But I have to say, my favorite character is Bom. It's technically not her drama, but it also kind of is. She's the driving force behind much of the story and character progression, and her story arch is a deeply emotional one that spans the entirety of the drama. It's ultimately through Bom's actions and words and presence that the rest of the characters are pushed into growth and reconciliation and developing new hope for the future. If no other reason compels you to watch this drama, then watch it for Bom.

Word of warning: The last two episodes are a sob fest, so have your tissues at the ready.