This book took me by surprise. It was so much better than I anticipated and I'm so sad I didn't read it sooner. I love Leigh Bardugo's writing style, and the way she crafted the characters and plot of this book is so good. 
(warning: major spoilers ahead)
To start with the characters, one of the coolest parts of the book, in my opinion, is that each character is more than they seem. The most obvious is the darkling. He seems to fall into the "bad boy with a heart of gold" trope, so we expect him to be gloomy but a good guy below that. I think that's why his character reveal as the big villain was so shocking. Usually, I can see those things coming, but this one took me totally by surprise. Looking back, it makes total sense and Leigh did a great job setting it up while still keeping the twist impactful. This applies to Genya too, seeing as she appeared to be an ally of Alina, but was just following the Darklings orders. One character that I didn't really like was Mal. I'm not usually a fan of best friends to lovers, so Mal didn't really stand much of a chance, to begin with. Then they did the "only love you when I lose you" trope, which I hate. I blame part of my dislike of Mal on the fact that Alina was manipulated into hating him, so from the reader's perspective, for a lot of the book, Mal isn't a great guy. When he comes back it's a little better, we can actually see that he's not that bad. I do think it's annoying that he only realized he liked Alina once he lost her. Overall I think that trope is dumb and sets a bad president, how much can you really love someone if you have to lose them for you to realize it. If you really love someone you should know when you have them in your life. I think I might grow to like Mal more, but it depends on how his characterization goes from here. Finally, I really liked Alina as the main character. I liked seeing her grow from the beginning to the end. I wish she was a little more confident in who she is and self-sufficient. It's kind of frustrating knowing what she's been through and then seeing her be so downtrodden with a little bit of struggle. I understand she is in a totally new setting, with a completely new identity thrown on her, and I think that definitely warrants struggle and doubt, but I would've at least wanted to see her be a little more set to figure out her powers. Especially given she was so determined to help all the people by getting rid of the fold. Once she did get a hold of her powers, she was really awesome, but I wish she could've been powerful as just Alina not only when she became Alina the sun summoner. 
I was really taken with the plot of this book. I could not put the book down. I think Leigh did a great job moving the story along without it dragging while still having an in-depth world. Overall the plot moved at a great pace and everything made sense. I think Leigh also did a surprisingly good job incorporating the Russian-inspired elements. As an Eastern European, it can be hard to read stories written by non-Slavs that try to incorporate those elements. What Leigh did well was taking the inspiration of the culture but creating her own world and story. I never felt like the cultural elements were misplaced or misunderstood, and it didn't drag the story down, but rather added an interesting element to the world of the story. I cannot wait to get to the rest of the series to see how Leigh builds on this world and the plot. Overall, I would recommend this book over and over, such a good read!