Massive spoilers. Coming from someone who enjoys spoilers, don't read further. I regret spoiling the ending for myself.

Seriously.

I mean it.

Really really really good. The relationship between Hee Do and Yi Jin is probably the most natural one I've ever seen in a drama. Even the inevitable separation--as heartbreaking as it was. The last fight might actually be my favourite part of the drama despite the way it made me feel. The way they started calm and spiraled into being hurtful was so real, as was their regret afterwards. Overall one of the best dramas I've ever watched, but it would have been better had it not been told from the perspective of a daughter learning about her mother for three reasons.
Firstly, the present Hee Do is so different from the past Hee Do to an extent that I can't just classify her as simply being more mature, so the switch between past and present may feel a bit jarring.
Secondly, we know nothing about her husband. We know so much about Yi Jin but we don't know the husband's name, his age, his personality, anything. We go through so much to understand her relationship with Yi Jin, we see them grow closer and more in love, and we see them fall apart, but how did she move on to her husband? How did they meet and grow closer? What's special about him that allowed her to love him when she's already experienced the kind of love she had with Yi Jin? What's so different about him that he and Hee Do stayed together and had a child?
Lastly, her daughter is completely immaterial to the plot. The only thing she did was say she didn't want to do ballet and then say she wants to do ballet. This could have--and should have--just been a straight forward coming of age story about Hee Do, fencing, and her first love.