I am not sure why this has so many negative reviews, but I suppose everybody relates/sees things differently and that's perfectly fine. Please don't hate on people for having a different opinion than yours, it is important to respect each other's thoughts even in the free world of the internet. :)

I really liked how well produced was this drama. The frames and camera shots were very raw and honest. The whole story itself had a mature kind of vibe to it, even though it was a romance. The plot did not focus only on the love story of the main leads. It also showed their own progression and growth, I guess that's what you call a coming of age type of drama. It helped the plot develop organically.
I also liked the idea of the past-present alternation, how it started with something that happened ( a momentum) and how it all goes smoothly to show how the story got there. It is indeed a really interesting POV. It makes the viewer feel included in the plot as to say "you are here with us too. Let's see what happened so far. How did we get here?".
I will always have a soft spot for Park Bo Gum, but he never ever disappoints. I believe he is one of the most expressive k-actors out there. He looks genuine when he cries, and I mean not the ugly forced crying that looks like someone splashed your face with water. But the emotional sensitive one. I just love how he takes a character's story and just makes it his own life. He gets immersed in the story and the role. And no matter how "serious" or dangerous his character may be, he will always look like soft marshmallow boy. This world does not deserve such a pure soul.
But, his dramas are all worth watching. No matter who he co-stars with. His aura will make everything shine gold.