I've watched this drama 3 or 4 times and I never get tired of it!!
Bong Soon's story is so captivating that it's stuck in your memory forever. The acting, the OST's and the whole scenario is so perfect that it makes the story of mystery and lots, lots of romance just PERFECT.
It is, without a doubt, one of my favorite dramas of all time and I highly recommend it.
In addition, all the events that happen throughout the drama make the characters grow a lot, which further improves the story and makes it not tiring to watch.