I will recommend this drama to only those who can handle really sad endings. Finished this drama 3 hours ago, but still, I am not able to remove it from my mind.
No words can express how much I hate the ending of this drama, it made me hate the protagonist Hae So ("IU") so much that I think I even stopped liking IU because of this. Also, it made me feel so sad for The King Wang So ("Lee Joon‑gi") that I felt like my heart was breaking into a million pieces. This drama has the saddest ending among all the dramas or movies I have ever watched in my life and that is saying something because I have watched like a gazillion dramas by now.
Spoiler:
Not even a single person finds happiness in the ending among such a huge number of people in the main cast.