I read the sample chapters for Duainfey at the Baen website months ago, and enjoyed them enough that this went on my list of "to be read when it comes out." I saw it in the bookstore, but fortunately shied away from paying the hardcover price for it and got it in the September Webscription from Baen instead. I don't want to think about how mad I'd be if I'd paid hardcover price for this book.
The book gets two stars [revised to 3 stars, see below] because I did care about the characters and keep reading to see what happened to them. It only gets two stars because I didn't get to see anything very interesting happen to them. "THE END" comes at an arbitrary point where nothing much has changed, nothing has been resolved, and no questions have been answered. The two main characters, who are followed in alternating chapters, don't even meet by the end of the book!
Duainfey, unfortunately, isn't a full book. It's an introduction to the next book, and it doesn't even end on a cliffhanger, it just cuts off.
As other reviews have mentioned, Rebecca dodges a horrible marriage by going off with a fey gentleman. Then the story takes a steep dive into gratuitous sexual degradation, which starts out relatively mild and escalates into gang-rape by the end of the book. Rebecca is a supposedly "willful" woman whose willfulness seems to be simple impulsiveness with no actual self-determination. I kept hoping to see her start to develop some sort of goals or desires of her own, but she didn't. Instead of plotting escape when she's unhappy, she begs, and any resistance she offers seems more like a haphazard impulse than actual defiance.
Regarding the other main character: through flashbacks, the reader is shown that Meripen is horribly traumatized by the torture and degradation he and his (now dead) lover suffered at the hands of humans eager to steal their magic. That's basically the only role he plays in this book, except to give a little background to Sian, who in the last page or two helps Rebecca win free from her (partially self-imposed) sexual slavery. (I'm not really sure why Sian gets involved, she just does.) He hates humans for what they did, until he finds out that there are humans living on his cousin's lands, and gains an inkling that they may actually be decent people. This doesn't seem like too much of a struggle, since the trees are willing to vouch for them. I suppose it might be counted as a small change, but one that I would anticipate at an early chapter-end, rather than at the end of the book. He doesn't get the chance to do anything with his inkling, or even turn it into a full change of opinion.
In retrospect, it seems blatantly obvious that the only purpose of this book is to subject Rebecca to enough abuse at fey hands to make her damage and trauma equal to Meripen's, while letting the reader know how damaged and traumatized (at human hands) he is already. Once she's been sufficiently abused, the book ends, presumably leaving any actual plot to occur in the second book.
It is possible to write duologies, trilogies and series where each book has a plot that wraps up enough questions to satisfy the reader while leaving enough loose ends to entice them into reading the next book. Just look at the books of Mercedes Lackey, Patricia Briggs or Holly Lisle for excellent examples of this.
Or, as the authors have chosen to do with Duainfey and Longeye, you can take one long book, hack it in half and sell each half as "part of a duology" to make more money, with no care for whether the first book leaves the reader with the literary equivalent of blue balls.
[Edit: I've learned a lot about the market in the past couple of years, enough to have a lot more sympathy for the authors and how little control they may have once a book goes into the publisher's hands. I haven't read Longeye, and I wouldn't suggest reading Duainfey without the sequel close to hand, but I'll reiterate here that I did care about what happened to the characters, and if you don't mind the sexual abuse in the content, you may well enjoy the two books together. If the book was simply bad or boring, I wouldn't have been disappointed enough by the ending to write this review.]