I enjoyed Ysabel. I found it hard to stop reading, in fact, and read it in a little over a day--although it's not a long book at all, quite short by Kay standards.
The story was compelling for me, and I liked the characters and the setting (Provence). It was interesting to see a couple of characters back from the Fionavar books. I thought that in many ways Ysabel felt like a continuation of Fionavar: some of the same characters are present, as are similar themes and events.
I think that if a person really enjoyed Fionavar, they will like Ysabel too. But those who disliked Fionavar will likely disklike this book too, because it has the same epic romance feel to it--some might even call it cheese. :)
My complaint about this book is the dialogue. Kay seemed to be trying hard to create a strong contrast between the ancient world and the modern, so he made sure to include lots of mentions of iPods, cellphones and Google, and filled the modern characters' dialogue with TONNES of "likes" and "whatevers." The dialogue was so slang-filled at times that I had to cringe. And it definitely didn't feel like Kay, whose prose is normally lyrical and beautiful.
I also noticed several times that these painful conversations seemed to go on far too long. There were many times when I wanted the story to move forward, but had to instead slog through several pages of characters talking about what had already happened. I think the book would have been improved by having less talk and more action.
But thankfully, it didn't take too long to reach the end of this story, and the end is fabulous, and surprising.
Overall, a good book. I'm glad I read it, but I'm sure very few readers would say it is Kay's best.