PROS
1. Really really good villain (made it really enjoyable for me). The villain had a well-rounded character which made me understand her more yet hate her.
2. Really interesting story (even though there was some dragging parts). I couldn't stop watching I kept wanting to know what was gonna happen next.
3. I really enjoyed the amount of action I witnessed.
CONS
1. While I love Yoona, the character that she was given made it slightly disappointing for me. Her role in the drama - running away, trauma, tidbits of cute moments. I really wish they gave her character a little bit more personality so that we can see her character and relationships grow in a smooth manner. Due to the lack of character growth, her relationship with the main character seemed a little forced and confusing. I noticed a few comments saying that Yoona's acting was bad. I personally think it's just because of the character she was given (I've seen some of her other acting stuff and she did well in those).
2. Some of the fighting moments seemed a bit weird/unrealistic to me but, this is not really a major problem unless you're really into action scenes.
Overall, I mostly enjoyed watching this and I would recommend you give it a try before making any decisions.