Age and maturity difference was a deal breaker for me . .

Unlike other vampire/immortal concepts, Kim Shin looks 40, acts 40 and he has the power and wealth of someone who is 40. Meanwhile, Ji Eun Tak looks 17, acts 17 and she has no power or wealth at all to her name.

The fact that Ji Eun Tak is world-weary due to a traumatic life, and Kim Shin is currently on a playful/mid-life crisis streak does not solve this fundamental problem. All the cinematography and production budget in the world couldn't solve this problem.

I really liked the character of Wang Yeo (the grim reaper), and his scenes somewhat redeemed the show for me. . . but not by enough.

I grade on other criteria as well:

Complex Themes - 3
In fairness to the show, I didn't watch far enough to see any complex themes. But, in the first five episodes it is your typical vampire/immortal romance (except more highly morally problematic than normal). There were hints of complexity developing.
Character Growth -3
Not up to the point I watched.
Nuanced Women -7
Ji Eun Tak is competent in her wise beyond her years but still has bursts of childishness. (The burst of childishness did NOT help with the romance hinted aspects).
Cinematography/Production Values - 10
It's really really gorgeous, with lots of real life sets and large production values.