Overall I can definitely say enjoyed this drama. The acting was good, the story was well thought-out and engaging, the action was great, it was full of twists and turns, never boring, and I generally liked the vibe it gave. They ran out of steam in the last episodes, and they're a bit more tedious as a result. Still enjoyable and I'd definitely recommend it if you're looking for a fun spy-ish/action/mystery drama.
I pretty much binged episodes 1 through 10, which were really great in my opinion. It was fun, fast-paced, surprising, and intriguing. I felt for the characters and couldn't wait to see what would happen next. The writing was great, each new thing working perfectly and bringing new life to the story just when it needed it. There's a bit of a change of atmosphere early on that's well-done and very intriguing. The mystery unfolds naturally and with good impact and left me looking for clues, trying to predict what would happen next.
The last episodes were still good, but the writing was less clear, I got a bit confused or couldn't see where they were going with some things. It wasn't to the point that I didn't understand anything anymore, but it meant I was more passive, no longer trying to guess what was happening because I simply didn't have enough clues or enough understanding of what I was given. The ending was not bad but a bit disappointing. After everything the characters go through, the resolution arrives quickly and pretty easily, and there's very little impact to it. I was honestly worried about how they would fit the real ending within the remaining time, until I realized that was it, it was just a bit of a non-event.
Sadly that's a problem that's not present only during the ending. Some reveals, twists, etc. did not lend very well, but they were mostly minor things, and mostly it wasn't a problem with the idea itself, but the way it was done, it's like the drama didn't pay that much attention to the twists it set up, didn't give you time to savor them, it just moved on to the next thing like there was nothing worth noting. It's frustrating to see the drama achieve that tension and nice twists consistently for most of its length, and then falter in that very thing near the end, but I'm guessing it might have been due to them running out of time in the last episodes.
I really liked the main characters, especially Seohyun's and Go Kyung Pyo's characters, however the secondary characters were a bit forgettable.
All in all, it was not my favorite drama ever, but it was definitely nice, I had a good time with it. With a few improvements on the last quarter or so, it could have been really great.