The main hook of this dramas was its music score/OST there was not one bad song in the title. I have seen a lot of Kdramas and for every one that has one or two memorable songs there are very few where every song not only works, but is worth listening to again and again, even my mom who doesn't watch dramas remarked on how pretty the songs were.
The plot is nothing unique, it really is a typical ghost drama, but the chemistry and acting by the leads makes it memorable.

The finale left an open end for a season 2, I would watch it.