This well written evocation of a vaguely Celtic fantasy world is an excellent addition to the usually reliable "Masterwork" series, and is a memorable, thought provoking work.
Its story is not so much predictable as inevitable, the reader can probably predict how the various characters will end up, but this is not a criticism, rather the opposite : their motivations and personalities are established clearly and strongly.
The magic here works in a similar way to that in Ursula le Guin's fantastic "Earthsea" books: naming things gives wizards power over them. It is not as clearly defined as in the le Guin books, making it easier for McKillip to use sorcery as a plot device, since what it can and cannot do is never really spelt out. This has the detrimental effect, however of the magic system never being as believable as in the "Wizard of Earthsea."
This novel is sharp and fast moving, very pacifistic in outlook, very feminine almost, if such a description is not too politically incorrect. My only real problem was with Sybel's behaviour after her duel with the powerful rival wizard. Keeping her motivation secret from Coren, but not from his brothers seems pointless in plot terms and somewhat out of character.
Lord Dunsany is another influence, obvious in some of the elaborate vowel heavy names of places and people...surely the wizard named "Myk" is a mistake though, it looks good on the page but try reading aloud : the mighty mage Mike just doesn't sound right somehow.