This review covers the entire Boreal Moon trilogy (but the score above is for the individual book - should be 2.5, by rights).
I'm a big fan of the Saga of Pliocene Exile, and I also enjoyed the Galactic Milieu. So, since I was unable to get the books one by one as they appeared, I bought the entire Boreal Moon trilogy in one go, excited for a real treat.
This wasn't it. From the start, the first book failed to capture my interest, but eventually, I forced my way into the story, and after a few chapters, it got better. But not much.
The series relies on complex politics, and simplistic individual motivations. The evil foe (the Salka) are a caricature - literally stupid, evil, slimy, green, tentacled baddies - sidestepping the fact that they somehow created the special magic sigils the entire trilogy depends on. Worse, the entire story ignores the fact that the slimy creatures are the aboriginal inhabitants of the island, and that humans displaced them through conquest. The fact that they want their land back just proves their evil nature. Good creatures who want their land back are fine, though.
The omniscient narrator tends to forget that the characters are not (meant to be) omniscient, and central figures keep picking up key bits of information almost at random. The magic system is barely examined, and is highly inconsistent - for example, "windscrying" (clairvoyance) is widely used, but virtually no one takes even simple precautions against it. This means that all sides can easily pick up opponents' plans - except when scrying mysteriously doesn't work (or isn't considered) - all too apparently for the convenience of the author. Finally, the resolution of the trilogy is very much ex machina.
May relies here heavily on an omniscient, yet coy and perpetually vague oracle/fate. She used this same technique to slightly better effect (though near-equal reader frustration) in the Galactic Milieu books. Having now read all her major works (including parts of the Trillium and Rampart Worlds series), I can say that she was at her best in Pliocene Exile, when her voice was fresh and the setting unique. Much less successful, though still interesting in the Galactic Milieu, which built on part of the same background. The Boreal Moon trilogy, however, uses the same techniques in a fairly standard-issue fantasy setting, and it just doesn't work.
The trilogy is slightly dull and convoluted in the first volume, but still worthwhile for May fans. The second volume (Ironcrown Moon (The Boreal Moon Tale)) is substantially less interesting, but does carry the story forward. The final volume (Sorcerer's Moon (The Boreal Moon Tale)) is a very hard slog indeed, and worth reading only for those who just can't stand to quit a story part way through.
If you enjoy Julian May and epic fantasy, skip this series.