Just finished this after learning it was nominated for a Hugo Award. As a fan of the Temeraire books, I already had pretty strong expectations going into this. I liked almost everything about the book, but there wasn't a whole lot I absolutely loved. I think 3.5 would most accurately sum up how I feel, but I'm rounding down to emphasize two things that bothered me about the book. 
 The world is a fairly unsurprising medieval fantasy setting, but with a refreshing Slavic touch. The plot did drag a bit when the heroine Agnieszka leaves her magical valley and runs around the capital a bit, but otherwise it was fun and easy to follow. Agnieszka was relatable and sympathetic, but in a fairly standard way - clumsy, not as pretty as some of her friends, unsure of what to do with her life. I feel that way too, so I could relate! But we do see these kinds of characters a lot in fantasy, so if you're dying for different kinds of characters this might frustrate you. 
 There are really only two things I would actually single out as real problems with the book. First, the romance (between the heroine and her captor-wizard The Dragon) wasn't quite as convincing as it could have been. The Dragon is fairly unpleasant at first, and just... stays that way, at least in terms of attitude. As the story goes on his actions become more sympathetic, but pretty much all of his social interactions with Agnieszka paint him as an impatient jerk, from beginning to end. I don't even recall him being made out to be especially handsome, or otherwise superficially appealing. It's not at all clear what Agnieszka sees in him as a romantic partner, though at least her feelings of attraction to him are quite well written. 
 The second thing is that the magic system is extremely haphazard. At first it seems fairly clear what's going on - there are magical spells that wizards and witches speak, they drain energy, etc. But the heroine seems to be able to sing her spells into doing just about anything she wants, with very little training and without a clear explanation of what's going on. The author makes an attempt to offset this by showing that Agnieszka is very bad at "traditional" spellcasting, but that doesn't feel like a problem when Agnieszka's magic seems in every way superior to what the rest of the witches and wizards are doing. 
 Luckily Agnieszka's own absurd power is offset by the evil Wood, an incredibly powerful villain that's quite effectively portrayed and refreshingly different to what I'm used to seeing in fantasy, so this magical imbalance to me felt more like a mild annoyance than a truly distracting issue. Also, props to Kasia, Agnieszka's best friend and a very fun twist on the old damsel in distress trope. I ended up much more invested in that side character than I thought I would be. 
 Overall, this is a fun fantasy novel that could have used a much more compelling romantic interest for the heroine, and maybe, if you're a stickler for such things, a more grounded magical system (though in fairness, this is supposed to be a fairy tale, and fairy tales generally have very flexible magic).