This book...let's just say I have a lengthy review to write at some point, but for now, I'll condense my mixed feelings as best I can.
In some ways, this book was five stars all the way: it has a refreshingly strong engineering focus; in the first half, at least, it details the challenges of terra- and ecoforming in satisfying depth (in this respect, it makes up for all that the Mars
trilogy lacks;) and it features an interesting use of a developing AI as narrator. It also raises many of the more important philosophical questions underlying space travel, colonization, and technological progress in general.
But in other ways, it earns two stars at best: the characters become progressively more wooden and dislikable as the novel lengthens; and the scientific implications and conclusions drawn by the protagonists (and presumably the book itself) are unwarranted, specious, and emotionally inflammatory on an almost sophomoric level. Instead of just leaving those great philosophical questions explored and up to the reader to answer, Robinson seems to pick a stance almost at random (there certainly isn't enough evidence in the novel to support it) and spends the last third of the book making a tedious argumentative appeal to emotion through his characters.
The end result is...very mixed. I'll give it three stars for now.