I just binge watched this show in 2 days (yes I have no life). I have a lot to say so bear with me. Okay guys *cracks knuckles*
Note: The plot seems promising at first but it eventually turns into your average kdrama plot. My rating is not good because of the plot but because of the brilliantly well-written characters and the amazing acting by the main cast. It's a one time watch. Have fun watching it and don't take it that seriously. Before watching it throw out all your expectations and assumptions (especially if you've read the book or watched Dangerous Liasions (1988) or Cruel Intentions (1999).

First of all, the cast is amazingly good. The acting is phenomenal. Woo Do Hwan did a great job portraying Shi Hyun. He had the whole melancholic, guilty and conflicted vibe of the character down to a T. Also he oozed sexiness and cuteness as required by the scene. He was perfect to play this character. Now, Moon Ga Young was the one who stole the show. She portrayed Soo Ji's character so perfectly. Joy also did well. I didn't expect much from her but she exceeded my expectations. All the other actors too played their characters very well.The characters are so very well-written. They are complex characters with deep flaws and fears. They pull from you the reactions they desire. The characters are multifaceted and you can't just only hate or love them. You come to identify with them, sympathize with them despite their faults and feel what they are going through. Honestly though, it's hard to find such truly well-written, complex, flawed characters that truly seem human.The plot is loosely based on the French novel 'Les Liaisons dangereuses'. I've read the book and seen the American adaptation. So I was very apprehensive to be honest. 4 episodes in I got so attached with the characters that I mentally prepared myself to bawl my eyes out. Anyway, it's a very loose adaptation. The plot was very good in the beginning. The whole mystery factor coupled with suspected betrayals and twisted games was very interesting. It gave you high hopes. Though the plot seems obvious and the twists predictable but it was still enough to get you hooked. Though I was sorely disappointed when they didn't turn out as I expected. I expected it to be more dark, twisted and gritty. A true tragedy. It wasn't though. It started lagging in the middle. Most plot reveals were milder than we had been lead to believe. It picked back up in the ending. I was really surprised (in a good way) by the ending because I was expecting something very different. But the ending was good.Honestly, what made this show worth watching was the amazingly well-written and beautifully portrayed characters. The acting was the best thing about this show. The plot failed to meet the expectations it created in the starting. But at least it wasn't that disappointing. Many kdramas have worse endings and truly lacking plots. It was still a good watch. It gets you hooked and you just have to see it through to the end.