Disclaimer: Kushiel's Dart is my very favorite book! This review will indeed wax poetic about its awesomeness, but I'll also give realistic feedback, as it's certainly not for everyone.
First, if you don't like reading about sex, but especially BDSM sex or sex as a spiritual/religious experience, then this is not the book for you. So if you're not down with a main character who is a masochistic god-touched courtesan-turned-spy who services men and women alike, pass it on by. But sex is not the main story here; it only adds spice to an otherwise sweeping epic fantasy plot. And this book is definitely well plotted!
The main drawback/weakness to the novel is that it does tend toward florid language. I personally would not go so far as to call it 'purple prose', but I know some who do. I think the language is absolutely perfect for the main character and contributes to a whole lot of wonderful world-building. Other books set in Terre d'Ange, with other main characters, are not so flowery as Phèdre's, and, in truth, I think the other two books in this trilogy are less so than Dart. In the first book, Phèdre is very much still a youthful, privileged 'Night-Blooming Flower', and her language reflects that. That's probably the biggest issue, which is only exacerbated, in my eyes, by the constant misuse of loathe/loath. Be flowery if you want, but, Blessed Elua, do it right! (I do think this issue is fixed by the second trilogy...)
There are a few other spots where the editor could have been a bit more on point. A few places where the King's granddaughter is referred to as King's daughter, a comma splice or two, that kind of thing. I actually didn't notice them until subsequent read-throughs because I was so caught up in the story, though.
My absolute favorite, from either a reader's or writer's standpoint, is and will always be world-building, and the depth and talent of Dart's world-building blew me away the first time and every time. The first page alone does so much, and it only gets better as the book progresses. This is set in an alternate-Earth, focusing mainly on Terre d'Ange – an alternate medieval France where the inhabitants are descended from angels. There is so much amazing world-building, both obvious and subtle, but Carey also deftly handles many other alternate-cultures, and you can see that she's done her research and honors each one rather than merely paying lip-service. They way they are all woven together is seamless and beautiful.
The characterizations are also well done. Phèdre could have very easily skirted Mary Sue territory, and yet she does not. All D'Angelines are gifted, and Phèdre more so than most, but she still has her faults and freely admits/deals with them even in first person narrative. The supporting characters are all just as intriguing, maybe even more so when we don't get nearly as much time with them. The list of characters is quite extensive, but those who matter easily stand out, and even those who don't are individual enough that it never seems like 'cardboard cut-out filler folk', except for perhaps the closely-linked Shahrizai (though we learn more of them individually in other books).
There is one over-arching plot that is extremely well-done, as well as many other side plots that keep us entertained and ever propelling forward in the story. The first time I read it, I zoomed through it, devouring it in my eagerness to find out what happens. In subsequent readings, I savored it much more and was able to appreciate the hints, foreshadowing, and tiny bits added earlier that play to the climax of this book or plots of those later in the series. The book is long, but each chapter and scene serves its purpose, and it's always entertaining. It's the first of a trilogy, but while it does set the stage for the second novel, it's still a fully complete and fulfilling read on its own.
Terre d'Ange has long been my favorite world, and those in Phèdre's trilogy have long been my favorite characters. But Kushiel's Dart is my favorite out of any of them. It's got everything an epic fantasy AND a kinky fiction book needs, and for a book that relies quite a bit on masochism and subservience, it's extremely sex-positive. TW: Rape makes an appearance as well, but it also does well with making the distinction between consent and non-consent. I don't want to say more without giving spoilers, but I will say that I think it was handled well. I still find the book to be extremely sex-positive and, blessedly, not reliant on heteronormativity.
It's not all sex and love. Intrigue, politics, betrayal, hand-to-hand combat, magic, war... Kushiel's Dart has it all, and it is fantastic.