Haven't watched the Japanese version of this so I won't be comparing the two :)

Story 8.5/10
The story develops pretty seamlessly and the pacing is pretty perfect, but that doesn't mean the drama doesn't have its problems. The drama carries through two distinct phases separated pretty much first half and second half so I'll frame my thoughts based on these two parts.
The first half involves the crime the synopsis depicts, and it's damn near perfect. What really steals the show the first half is the relationship between Jin Kang and Kim Moo Young. Just like Jin Kang, we have no clue what's going on in Moo Young's head but we slowly fall in love with him anyways! They have your typical "I hate you" turned "I love you" k-drama formula, but the way the drama does it feels more raw and less cheesy because Moo Young isn't your typical male lead. He's mischievous and you never know what he's thinking. I like watching a drama that has subtlety in what it's doing, it knows that it doesn't need to show in giant black ink what's going on for you to catch on but at the same time, keeps you just confused enough to stay curious and keep watching.
But once the crime mystery ends we enter into the second half of the story and that's where all the problems become more apparent. I can't really describe these without spoiling the story too much but just be expecting these things. First, the relationship gets kinda toxic because the situation is messed up. The side characters that were involved in the first half lose their purpose entirely in the grand scheme of things, so I can't help but wonder why they're even there (Seung Ah, Yoo Ri, Jin Gook's lady friend, Cho Rong, the brother's mean boss... need I continue?). Moo Young is probably the only character that goes through any development. Unfortunately, Jin Kang and Jin Gook don't go through any development, things just get revealed about them so you can be shocked about their past. Which is a shame, because the development with Moo Young is really, REALLY good. By the end of the drama, you REALLY feel for this man.
Also, you probably already know this going in but this drama is the definition of melodrama. Your emotions WILL get screwed with! :(

Acting/Cast 10/10
The acting is SO good, and it makes me so happy! It's a melodrama, but the main cast acts doesn't go overboard and they act their scenes out so that it really feels like how people would react if these things were to really happen to them. Once again, I think a strong point of this drama is its subtleties. In particular, Seo In Guk doesn't blatantly say what he wants or is thinking, he emotes it subtlety. This is not to say that during the dramatic parts of the drama he doesn't give a BANGER, because he absolutely does. During the final episodes, the acting is some of the best.
Also, can we talk about Seo In Guk and Jung So Min's chemistry? It's REALLY good. I was literally out here shipping them the moment they met.

Music 10/10
I'm not picky with my music, the music they chose was solid and did what it needed to. Especially OST #2 with Seo In Guk and Jung So Min, so freaking good! They're definitely songs I'd listen to outside of the drama (although I'm gonna have to wait a couple of weeks to calm down and power through my withdraw symptoms from this drama before I listen to it because I'll just cry if I do).

Rewatch Value 5/10
Wouldn't watch this again, but I wouldn't watch any melodrama again because I can't put myself through all the pain again. I'd for sure watch clips again to see the performances from the actors or any cute bits.