Generally I think K-Dramas are overrated on IMDB by about 1.0, so given that a 7.6 rating is considered "decent/good" which I feel is strange.
"A+" elite for visuals and effects, they're not overdone (like I think happened on It's Okay to not be Okay") but tastefully and artistically applied.
"A" for narrative pacing. I do think it could have been a few more episodes long, but that's only because I was left wanting more, not because the pacing was abrupt or anything, it felt like an artistic choice (whether it was or not, that's what it felt like) and made the drama feel precious while it lasted. A season 2 would make perfect sense here as the story does come full circle but at the same time feels like more can be told.
"A-" for story and premise. Story is very tidy, it doesn't try to be an epic or a masterpiece, but it's clean and cleanly executed. Sets out with an objective to be something, and does it very well.
"A" for acting. I was surprised at how charismatic Song Ji Hyo was on this, I've avoided her acting because I watch RM and felt I was too familiar with her to not see through her acting, but I've changed my mind after watching this. Solid acting.
Overall, again it doesn't claim or try to be a "masterpiece", but what it does try to do in being a cool and clean premise of a drama is "masterfully done" I would say.