I was very impressed with this book. Walton does a wonderful job of reworking the original Welsh legends into four stories. Naturally, she allows herself certain artistic freedoms, such as interpreting disputed aspects of the legends in a way that seems most plausible to her. However, her skill and her imagination make me gladly accept her interpretation.
The writing is probably the most beautiful I have read since Tolkien. It is rich in detail, vibrant, and poetic. A pleasure to read. The same is true for the characters, who really do come to life in Walton's book. She (re)creates gods and men, heroes and monsters, while at the same time exploring some of the recurring themes of humankind, such as love and loyalty, strength and courage, etc. The basis for all this is the same cultural background of the original Welsh mythologies, i.e. the fundamental conflict between the belief of the Old Tribes, in which women were quite independent and powerful, and the New Tribes, in which women are inferior to men and the role of women as 'creators of life' is slowly forgotten.
At the same time, I don't think this book is for everybody. If you enjoy contemporary fantasy with a Celtic background along the lines of Katherine Kerr, then you may be disappointed by this book. Not every subplot is pursued to completion, not every character is described fully. Walton implies as much as she tells us. The language is as much poetry as it is prose. Like I said, it's beautiful, but read it for what it is!