It was fantastic and thrilling. At least the first half.
The story was good. It looks too obvious at first, you think you already know everything and may think it's boring, 'cause why watch it, when they tell us everything, but no, the things here are not that simple. You have to watch it, because nothing is what it looks like. Because of the twists and turns, it kept me in the screen all day, not to mention the OST (my favourite was that one with the organ), and the acting (especially Lee Seung Gi's, he was just fantastic).

But as we went on the story, and as we went nearer and nearer to the end, it became a little draggy. Things became more clean, the story became more predictable, the plot wasn't that dark and thrilling like before, the twists weren't surprising anymore. I suspected a way darker ending and deeper things behind all the happenings, and the last episode was just like they were in a hurry, and felt like they 'had' to answer all the questions in only ONE episode. In the last episode, things just happened one after another, like it wasn't whole, like they torn it apart to little peaces to answer all our questions and solve all the leftover problems, which they could've done better if they did it little by little maybe in the last 2-3 episodes.

And the ending - after I watched it, I just felt I really, really want to cry. It was unfair. I wanted something else. The person (I don't want to spoil) who recived the punishment, wasn't the same person anymore who DESERVED that punishment (if you watched the drama, you know what I mean), and it just felt so, so unfair, even if I know it was the proper thing to do.

After all, I gave this a 9, because I'm in deep love with the first half. It compensated me for everything they've done in the end. I've put this in my "TOP" favourites list, and it's possible that I will watch it again some time later.