My biggest praise of Witchmark is also my biggest criticism: this felt like a debut original novel from an author who has written a lot of fanfiction, and who deeply understands what makes fic appealing and how fic works as an artform.
So, the praise: at no point did Witchmark feel like a chore. Every single page was a delight to read, and I looked forward to it. I was rooting for the characters the entire time, intrigued by the murder mystery, and viscerally engaged by the drama. There are enough little details to make the world vivid, while the prose remained light and streamlined. In a lot of ways this book is a very fast car: you can see the engineering that went into it while laughing out loud at the thrill of going very very fast around sharp turns.
The flip side is that, well, I really love Sherlock Holmes stories, so this isn't really a criticism, but this is definitely a Holmes story, to the point that I can easily draw comparisons between Witchmark and various other iterations of Holmes and Watson. It draws more on Conan Doyle than on BBC's Sherlock: it's got the class issues, the landladies, the bicycles, the compassion and the sophisticated murder mystery plot; the sexual tension is understated in the classically British manner, and it doesn't have particularly snappy dialogue, the queerbaiting, or the complete failure to understand its genre. This is not to say that the story is unoriginal so much as to say that the originality wasn't really the point. I loved this book because I love Sherlock Holmes.
In fact, I like this cosmically political version of Holmes and Watson even better than I liked Gaiman's "A Study in Emerald" (or the remix of Gaiman's story by M Leigh, "A Study in Midnight"). Polk's injured-doctor-home-from-the-war is the best I've seen: I love the turn from surgery into psychiatry, and Miles' family issues make his character more interesting while they drive the plot. Her fey-genius-moonlighting-as-a-detective is a little less exciting, because we see less of his backstory and inner life. You can see fandom's influence on Witchmark in the clear and matter-of-fact queer romance (which is better than any TV or movie version of Holmes and Watson, so far).
I would have liked to see more analysis of the consent issues around the magic, and I especially would like to see more of the political implications of the resolution of the murder mystery. I won't spoil anything, but I tend not to read steampunk or fantasy set in alt-British Empire, because I think it's harmfully imperialist. The American empire uses dreams of the old British Empire to justify our political and military interference with other countries and our cultural self-centeredness. I was hopeful, from comments I'd seen from other readers and the way the book was marketed, that Witchmark would problematize the empire a little more than it did. Perhaps that happens in the upcoming sequel.
n.b. I got my copy of this book via the Tor.com eBook of the Month Club, where they just give you a free ebook. It's lovely, I recommend signing up for it because there's literally no downside, they don't even spam you with marketing emails.