It's a brilliant, incredible yada yada show. The perfect casting meets with with the best creative talents the kdrama genre has to offer. There are plenty of 10/10 reviews very accurately depicting the awesomeness of the show, so I'll keep it short.

The reason why it's not 10/10 for me is because I have gotten into kdramas specifically due to the fact that kdramas tell a somewhat coherent story in 16-20 episodes & usually they are light on the filler and the good ones give the audience closure. This show has been written with a second season in mind & it ends with a cliffhanger. The cliffhanger itself is not that offensive, but two of the three most important storylines are hanging in the air, unfinished. So yeah, the last episode was quite the maddening experience.
I really wish Netflix stopped trying to influence how kdramas are made..