First of all, I love this drama.
It has everything I cherish and want in a drama. Good cast, interesting story, balanced mix of genres and a catchy OST comes as a bonus.
Then why not a 10 rating? Let me try to explain (without much spoilers).

I had trouble getting into the story. It was a mixture of something in the past and the present and by the first couple of episodes I was lost with a lot of questions. They got answered but the beginning was still a bit of a struggle.
Things get revealed and the pace of that happening is good, well until the end. I think they rushed it a bit too much. We get to see the couple and hints for the other characters, telling us how their life might continue but it didn't feel enough.
I guess I was simply not ready to let go yet and by the second last episode I felt ready for a big coup, what I've got was a few minutes long and just didn't do the story justice.

The acting was really good and there's nothing to complain about since we're talking about well-known actors here who were part of many dramas. I especially enjoyed the chemistry between Ji Chang Wook and Park Min Young and with Yoo Ji Tae. Kim Mi Kyung is a goddess, she made it to my top favorites within a couple minutes. xD
But also the support characters stood out. Gosh, Jung Gyoo Soo gave his character a creepy aura while Park Sang Myun and Woo Hyun were such a heartwarming and funny duo.

I still have the theme song of the drama in my mind. The OST is really catchy.

Rewatch value for me is middle. I probably won't come back to watch the whole thing but I surely want to rewatch the cute couple moments, action scenes and funny moments.

Overall I enjoyed the drama and just think they could've done better at the beginning and end. Maybe give the end a whole episode and not just like 10 minutes to wrap everything up. Then it would've been perfect.
I certainly recommend it though.