The best thing about this for my was Krystal and her character and Song Seung Heon`s looks (not his acting though).
It is a light show but also very shallow. In therms of characters and story. Especially the former lacked nuances for me (which is probably why I didn`t feel Song Seung Heon`s performance/character that much).
In terms of action/plot: It got really boring since they used almost the exact same routine for every event. It was/become very predictable and I wasn't a single time surprised by plot "twists".
The villain were also these comical cartoons cliche villains that were only there to enjoy when they finally got kicked in the nuts.
And not much fight "choreo" to see either they went just style confusion camera wise and so you could mostly only see some disjointed stunts. But it was okay.

I really don`t agree with the sentiment "This drama is considered a must-watch in many ways". Can`t think of a single wey/thing. To be precise I think it's very overrated (at the moment 8.4) compared to others in the genre. But the same is true for me about "Come and Hug Me" and "Mad Dog" so... (Though this one is in the middle there).
My expectations are probably to high when I compare them to my favorites.
Though considering thriller is one of my favorite genres - frustrating.