Summary: disappointing, especially since the reviews were generally good
Pros:
The story was more unique than some and had good elements of mystery
The cast was amazing; it's not their fault the writing was terrible. They all portrayed their characters very well, especially Ji Chang-wook and Kim Hyeong-gyoo
Cons:
Incoherent: I am happy to suspend my disbelief, but this drama asks way too much. A lot of really nonsensical things happen that are so unrealistic it's distracting.
As others have mentioned, this drama focuses on romance to the detriment of it's storytelling. But even the romance is disappointing. The characters seem to really not have a healthy relationship, and I don't think Bong Hee is mature enough to be in any type of relationship, even at the end of everything. And the Ji Wook is way too good for her anyway. She is annoying at best and a complete idiot most of the time, which I hate because Nam Ji-hyun is amazing. I really didn't like the way they treat each other and would have actually been happier if they had not wound up together.
There were a lot of opportunities for explanation and side plots, but they were all wasted, so it ends with a lot of unanswered questions and unsatisfying "resolutions."