Starts fast, but slows towards end.
I would recommend this book with some reservations. For SF readers, the book is a "must read" by a pair of the great names in the genre. The pluses are intriguing ideas and polished writing--but it was difficult for me care about what happened to the characters after 200 pages. The book becomes more of an essay (or allegory) than a novel. Character and pacing were sacrificed to concept. The book might have been better if an editor could have convinced these two legendary authors to cut out 100 or even 150 pages from what is probably their last book (Clark died in 2008 and Pohl was in his 80's or 90's)--but they wanted to get in their last word. The writers set the stage very well in the early chapters, but they didn't sustain my concern for their characters as people, and the pacing did not propel the narrative. With those problems the book could not overcome the lack of suspense--the ending always seemed inevitable.