Chalk this up for another in the column of, "I really _wanted_ to like it, but."
I don't really know where to begin on this one. I think a lot of it comes down to that the book simply tried to do too much in too few pages. We have Ash herself, the mythology of faerie, the mythology of the hunt, the battle between the greenwitches and the philosophers, Ash's mother's story, the prince's story. . .it's an awful lot of ground to cover. In a different writing style, it might have worked, but Lo's prose is slow and dreamlike and distant. It's pretty, but it lacks the ability to move things along in the limited space she has.
It also lacks emotional intensity. I never really bought either of the book's romances. In the case of Ash's relationship with Sidhean, that may have been intentional. On the other hand, her realization that she doesn't actually want to leave the world for him is supposed to be a key moment of character growth; since I never for a moment felt that she _did_ want him, the change of heart lacks punch.
And then there's the ending, which bothers me more the more I think about it. I can read it in two ways: either Ash does still sort of want Sidhean, in which case the price of her having her heart's desire is that she has to spend one night with a man who she doesn't care for quite so much, or she _doesn't_ want Sidhean, in which case the price is that she's raped. In the former case, it doesn't strike me as a price at all; in the latter case, her complete lack of trauma makes no sense. Either way, it doesn't work for me.
There's still some solid writing here, though. I hope Lo's next effort will be better.