This K-Drama made me sob. I just finished watching it and my tears are not yet dry. This is the kind of K-Drama that makes your life feel empty after you finish. If you love tragic romance and screaming and ripping your hair out about how unfair life is, you're in the right place. There are sweet and funny moments in the show, but they mostly exist to emphasize the misery. And it works. Watch out for the later episodes, the show gets sadder the more you watch.
Kudos to all the actors and actresses, Shin Hye Sun and L especially, for incredible, emotional, and flawless acting. Even though Shin Hye Sun and L really shined in their respective roles, all of the supporting characters were also incredibly well-executed. I especially liked Do Jiwon and Bo-Mi Kim. (Also serious props to Shin Hye Sun for those ballet scenes, ballet is NOT easy.)
I love this show. It was so easy to become emotionally invested in the characters, and the incorporation of ballet into the storyline was really interesting and unique. The only criticism I have for this show is that I don't like the soundtrack. It was a little bit distracting for me, and I didn't like the actual music very much. However, this is just my opinion and others might tell you that they loved the soundtrack. Also the CGI was a little bad, but these are small, minor details in the show, and don't detract from it in any of the ways that matter.
Overall: Interesting and engrossing plot, well-executed, emotional acting, and well balanced cast. Highly recommended. Watch out for the plot twists (they gave me emotional whiplash), and don't forget to bring your tissues.