To everyone not knowing whether or not this drama is worth watching... Well, passing through the comments section should make it obvious. I watched it when it was still airing, and I was hooked from the very first episode. Seriously, it was so good I was scared to watch the last two episodes, I didn't want it to be over and yet they paced the drama perfectly.

The plot was refreshing and really well developed. I was impressed by how the backstory was used, not only to add depth to the characters, but also and essentially to serve the plot. Most (not all of them, thankfully) of the dramas I have seen only gave them a painful backstory so that we could either understand why the main guy was being so mean to the girl he liked, or why they were destined to be with each other.
In this case, what happened to them still had significant consequences. One of the reasons I liked this drama so much is that these consequences were never really solved, which makes the story more realistic. Going through something traumatizing does leave an imprint on you, the aftermath can be devastating and it doesn't just go away, as it is too often depicted in Korean dramas. It wasn't about being completely healed, but more about getting better and learn to cope with it. The main characters viewed the world differently because of it, and they understood each other in a way that probably no one else could. They stuck to each other in the hardest times, even when the other pushed her/him away.

Junho was a revelation in this role. He was really impressive, the way he portrayed Kang Doo's pain and distress left me in awe. He conveyed such strong emotions while keeping a blank face, I didn't even know it was possible. I feel like the drama wouldn't have been as good without him. Like, really, he was MADE for this role.

And Kang Doo will probably be my favorite character of all dramas. He was broken from the beginning and went through so much (okay, I may have a thing with problematic guys and painful pasts, I don't know why it just draws me in... And let's be honest I'm not the only one here...). It really made the story complete, because he dealt with things differently and didn't know how to love someone when his life and his mind were a complete mess.

In conclusion : if the depth of this drama isn't enough for you, then Lee Junho will make you fall in love with the drama for sure.