I liked this show quite a lot.
The plot is not 100% captivating per se (specially the subplots), which is the reason I can't give a 10.
But what is really engaging is the acting (and chemistry) of the two leads, a strong supporting cast (second lead syndrome alert, here!) and most of all:

1) the portrayal of a real relationship, the best and worst moments included, between two adults who have spent almost their whole life together, no sugar coating;

2) these people share believable kisses and love scenes, which is kind of refreshing considering most of the k-dramas show people over 30 acting like they're 12, as far as sex is concerned. That's what I wish more k-dramas could have: can you see, writers? You can show believable love scenes without turning your drama into a porn movie! It can be done! :)

Lovely OST too.