Let me be clear from the beginning: I absolutely love this drama and it will forever be one of my favorites :) The story, cast and music just fit so perfectly that one can really not put it into words. You have to watch this!

Story:
A boy and girl meet during an accident where a building colapses and the two are buried underneath the concrete. Both are rescued in the end, but this incident leaves them, and also many others, with different levels of struggles and pain.
A very sad story with a lot of struggles that turns into a beautiful life lesson. Seriously, I haven't come across a drama where such a sad begining turns into a wonderful peace of art, like it did here.

Acting/Cast:
All the cast members do a splendid job here, they depict their characters perfectly so you love and hate them in all the right places. Actually, there is no character that I hate in the end, although I hated him or her inbetween. Everything is resolved in the end. And then there are the main characters Moon Soo and Gang Doo, who are the above mentioned boy and girl. Moon Soo is played by Won Jin Ah and although she seems a little reserved in the beginning, she really did a great job overall.
And now, let me come to the star of this show. Lee Junho (of 2PM) as Gang Doo. GIVE THIS MAN AN AWARD! His acting is so perfect, so real that you can't help but fall in love with Gang Doo. Every little emotion Gang Doo feels is visible thanks to Junho -  wow, wow, wow! I didn't know him before, as I didn't watch Chief Kim, but now I've become a sucker for Junho (and 2PM xD) thanks to his acting here.

Music:
This drama has another great feature that lies in the music. Or rather, the lack of music. There are moments in this drama were there is no sound at all and you have no choice but to immerse into the emotions the characters feel. And that are the moments where it gets reaaaaally intense. Great! Also, if there is music, it always fits and is never too much. Another good point: Junho himself sang one of the songs of the OST and it's just beautiful!

Rewatch Value:
Yes, will definitely rewatch this at some point in time. Even if it's only to see Gang Doo's emotions!

Overall:
This drama deserves an award. Or better, a lot of them. It is really amazing and took my kdrama experience to a whole new level. An emotional rollercoaster that is worth all the ups and downs! It's simply a masterpiece. I bow my head to the writers, directors, actors, and all the other people who worked on this. Thank you!