Slow starting and a little confusing at first with the two separate timelines. I also found the past storyline a lot less interesting than the present day one at first. However, despite a pretty slow but interesting start, I thought the show got better as it went. It was definitely a slow burn, but I don't mind that as long as it actually burns (too many "slow burns" are just slow).
Despite the initial pacing, I thought the show never really got dull and the first bunch of episodes did enough to keep my interest. The last 3 episodes were great. Episode 6 really brought the past storyline together in one of the best episodes of the entire show.
Go Min-si is excellent in this. I really like her in Sweet Home and Youth of May, but seeing her as a villain really showed off her talent as an actress. She was a great villain. She also gave off these "I can fix her" vibes, which just made her even more interesting as a villain.
I liked the "good people in a bad place" type theme that ran throughout, I liked the scenery, the acting, the mood was creepy but not overdone. Overall, I thought it was an excellent show that maybe took a little longer to tell its story than I would have liked, but wrapped it up well.