This little-appreciated book is Greg Bear's best, in my opinion. Science Fiction it may be, but its themes are as adult and rigorous as any book in any genre. It is also very well written.
An air of melancholy and despair - as well as barely suppressed terror - carries right through from start to finish, as befits the situation set up in its predecessor, The Forge of God. Bear does not shirk the philosophical implications of the story he is telling. The humans and aliens caught up in the story are overwhelmed by the mysterious technology that surrounds them, and never fully understand what is happening.
The climactic battle is very exciting, and the ambiguous outcome a satisfying, well-rounded one.
The technological explanations make enough sense to be convincing, but Bear also makes economical use of little suggestions and implications to give texture to the weird, merciless galaxy in which the story takes place. The Braid aliens, too, are a superb invention. They seem completely ahuman, unlike most alien races in sci fi.