I'm reading very mixed reviews while I expected it to have hight ratings. So I will write my own.
I didn't think it would be good because I get tired of South Korean drama about rich families. I choose to watch it because it stars the tutor from Sky Castle. I ended up loving this show. The music is amazing, it makes every episode feel like you are watching a thriller. The atmosphere is like the architecture in the series From grand yet sinister, to transparant yet secretive. You feel like something unexpected might happen but it stays subtle, leaving things open for interpretation until the very end. It's very well balanced and detailed.
I felt sympathy at least once for every character. I don't want to give out any spoilers so I won't go into detail.
They could have picked a better child actor, but he's young so I'll forgive him. Sure the ending could be considered over the top, (and the police investigation was somewhat confusing) but it worked well for the storyline.