I'm trying to pinpoint which strength I want to list first. The characters? Amazing, three dimensional, super well-written (like most of Schwab's characters)! The story? Epic, fantastical, cinematic! Makes me almost think of Gaiman when he turns an ordinary human into a mythological hero living a life they write ballads about and tell tales of. I think my favorite aspect was that Schwab worked hard to really paint her words in this book. It's so much more florid and lyrical than her other books I've read.
Alas, the one downside was the pacing. It does start off slow. Don't worry, the pacing changes (I wouldn't say picks up) and you'll know it when you get there. And the end of the book's pacing is a bit of a whirlwind. A bit more things packed in there and I'd say it was too much too soon, but thankfully it is just a touch overcooked, but not burnt.

Spoiler

And +1,000 points for creating a Faustian character that keeps her end of the bargain and continues to play the game and doesn't turn into the moping, angry, depressed being that insists they're still human and their resistance just makes them unlikable. Also, none of this "I love this human, it's true love, it'll defeat the curse!" bull-honky. I applaud you Schwab with how you dealt with Adeline's personality, how she changes and grows within her deal with Luc and their relationship. I really, REALLY, did like how she dealt with Luc and Adeline's relationship. Although I'm always a sucker for Enemies to Lovers to Frenemies.
And another +500 points for Luc in general. For some reason I pictured black-haired Howl, which certainly didn't hurt my imagining of Luc while reading AT ALL.
And AND this is a complete novel. Not a series. +1,000,000 points from me for that alone.