First of all - this is like nothing I've seen before - very original, and interesting to see it incorporating mental trauma as a central feature of the plot. The exploration of that element was interesting, and the device to depict it was effective, but in fact most of what it says about mental illness and manipulation is pretty inaccurate. However, the entire plot depended on it, and mostly the madness was shown in a benevolent way, so I could forgive the nonsense.
There are massive plot holes, numerous plot twists, which I eventually found both predictable and a bit annoying. Seen worse though.
Acting was as always fantastic, music was good too. The star of the whole show was the developing relationship between So Yeol and K. It was both hilarious and touching and developed into a real (and unsurprising, really) attachment and eventual integration. Very nice. This drama made me laugh out loud, which is rare, but it was also a very forceful thriller. Completely unrealistic of course. The performances of DongWook and HaJoon were outstanding, and their chemistry unmatched - while the other characters were great, I wanted to see more of this pair together. Watch this for them if for nothing else.
The baddy is truly evil. Truly. Well imagined, and well-portrayed. At first appears very benevolent, but I spotted him almost immediately as the most qualified to do what was done. Not sure if that affects the enjoyment or not - he was actually officially "revealed" a little while after that - to my quietly smug satisfaction.
The details on exactly how he is brought down were not exactly clear to me - I don;t think that was me either - I think they really glossed over it. Quite a few other things were only just sketched in, and we had to take them on trust. Depending on the balance of those details with the forward thrust of the drama, that's a bit of a weakness for me. However that probably kept the drama quite short and avoided episodes dragging or being slow, so it may be not a bad thing.
Overall, though I found it a bit frustrating I enjoyed many aspects of this show - especially the two male leads. I would quite like to have a benevolent K in my life too.