Full disclosure, I mostly watched this drama for Taec-yeon and I am also now reviewing this months after having finished so some details will be lost--
I enjoyed watching this drama for the most part (until about two-thirds in) so that's what saves the score from being lower. The cohabitation trope, the classic fantasy immortal man looking for his girl, the historical elements and mystery of the flashbacks slowly coming together, the SML and his mystery disease-- chef's kiss. FL being different than the average Kdrama girl and actually kinda liking SML. Mystery girl with the matching face. It had some unique elements that made me enjoy it beyond Taec-yeon's acting (and face).

Honestly, I don't remember the OST. It didn't end up on my playlists, so I guess it was probably average.

SOME SPOILERS COMING

I watched it as it was being released, and waited each week for the next episode. At some point, that excitement faded. The intrigue started getting clouded by annoyances: the SML becoming kinda obsessive, the side characters being useless and ruining things, the weird cat guy who was just kinda there to be mystical and weird and then ALL OF A SUDDEN drops this very important lore he failed to mention before, the cheesy vampire antagonist, and honestly just the whole "I want love that makes my heart beat" message felt shallow. Like that could've been a joke because ML is such a goofy romantic, and instead it's legit serious sometimes but it feels corny. Cool FL starts to catch the corniness too, but I suppose that's what happens when Taec-yeon catches you.
Also that one side character's hair...

SPOILER WARNING

And then two thirds in you get a prediction of how things will end and I'm thinking "well this hasn't been the most unique drama ever, it's been full of tropes, so nah not happening they'll do the classic everybody lives except maybe a side character" and then it does happen. It's predictable in the worst way. The whole tone is just depressing after the revelation is dropped and then you just ride that wave thru a bunch of annoying "not telling you so I won't hurt you" tropes until the end. If you're gonna try to be a different Kdrama than the rest, be consistent. It just made his sacrifice feel like a noble idiot moment, she has no choice in it. Instead of character growth and them truly enjoying each other thru real communication (I know it's a Kdrama but still) it just feels like another grand gesture from him but this one goes horribly wrong. Questions left unanswered in other elements of the plot too. Ugh. Such a disappointment.