I'm only writing this because I got zero info when I went online to find out if this would be a happy ending or not - if this would be worth investing 20 hours into. If you're into melodramas, a lot of 'will-they-wont-they', action/fantasy, steamy kisses and implied sex, watch this!
The progress of their relationship was obviously frustrating because it's a melodrama and you know how long it takes for k-drama leads to make up their minds and say that they love each other. But hey, this was good! There were plenty of sweet and steamy moments to make the watching worth it.
Before starting the drama I read somewhere that 'everyone dies except PK' and that's total bullshit. Ma Wang, Son Oh Gong, Secretary Ma, PK, Sa Oh Jeong - they all live! Literally almost everyone lives except Jin Seon Mi, Zombie and the Prince possessing Alice. More importantly, Jin Seon Mi's soul even comes back from the dead for Son Oh Gong. The ending is a solid happy ending, or a heavily implied one if you're a pessimist. Ma Wang finds his son, Son Oh Gong gets to confess to Jin Seon Mi that he was actually head over heels in love with her even without the Geumganggo (that he takes off by himself in front of her), Son Oh Gong literally gives Jin Seon Mi the power of one of his eyes so that when he goes to the underworld he can find her easily and make her his completely. It was worth the watch. Go watch it!