I thoroughly enjoyed this book. Zahn's engaging writing style makes this book a real page-turner. He blends some elements of "hard" sci fi within a space opera and spy thriller context. The political/strategic conflict between the Pax and the Empyrean is fascinating, as is the scientific mystery about Angelmass that slowly unfolds as the story progresses. The philosophical issues (can "Good" be quantified? What constitutes sentience?) are thought-provoking and handled very naturally in the narrative. We don't find out the truth about Angelmass until the end of the book.
Also, the main characters are very believable and realistic, in my opinion. Kosta, Chandris, Forsythe, Telthorst, Lleshi, Ronyon, Hanan, and Ornina are all very human, and each seems to have very valid reasons for his or her actions. Their motives are believable and reasonable, given their backgrounds. Each character has his or her own fears/vulnerabilities and aspirations, and I found them all to be very memorable (BTW, I keep imagining Christopher Lee as Lelshi, due to his aristocratic, military demeanor).
I found the plot to be highly engaging, and I found the ending of the book to be very satisfying. One previous reviewer thought there were some loose ends, but I don't see it that way at all. I found the conclusion to be perfectly reasonable, without resorting to a "storybook picture-perfect" ending with everything neatly tied with a bow. On the contrary, I found the ending had more of an authentic feel to it.
If you're looking for a science fiction story with both a sense of wonder and action, this might be what you're looking for.