Bought this last weekend and finished it Friday evening. In one word: disappointing.
(Warning spoilers past this point!)
In more words, this tells the tale of generation ship on an interstellar colonisation mission. The book has a lot to live up to compared to his previous Mars trilogy and 2312. It's quite clear that this is not sent in the same universe as either (for one thing no longevity treatment), despite some common elements like the moving city on Mercury. The first third or so of the book detailing life on the (unnamed) ship is fascinating and reminiscent of the 'travelogue' scenes in the Mars trilogy. The world building is very detailed and helps you visualise the setting. Once the ship arrives at its destination (Tau ceti) things fall apart but in story and with the narrative. The titular planet Aurora tours out to harbour primitive life that proves deadly to the settlers, and this causes a brief civil war among the colonists as to what to do next.
This was this point at which I guessed what would happen next, and where the detailed ship description (with its two rings of 12 biospheres) went against the novel. As soon as it was declared a (roughly) 50:50 split between staying in the Tau ceti system and going back to Earth, I knew they would end up splitting the ship in half and giving each group and ring.
This was also the point at which missed opportunities start to appear, with the 'stayers' dropping out of the story- I'd have liked to know what happened to them. The 'returner's struggle against failing systems, ecospheres and cop failures for the next twenty years or so (none of which appear until they are under way again, conveniently for the plot). Another convenience is the sudden arrival of news of hibernation experiments, which despite the trials only lasting 5 years enable the surviving crew to hibernate the next 150 years until they get back to the solar system.
While the descriptions of the return journey (narrated by the ship's AI) and deceleration were beautifully told and thrilling, it was the final section dealing with the returnee's on Earth that proved the biggest disappointment. The message was firmly against the concept of interstellar colonisation, with one character outright declaring that future starship plans 'need to be stopped', and the tone one suggests we should focus instead on looking after this planet rather than finding new ones. Again there were more missed opportunities as the book simply ends with a trip to the beach, when some of the ideas (the returners struggles to adjust to Earth, arguments about future missions, what is going on with the other colony missions mentioned, etc) mentioned briefly not being followed up.
Overall while the book had some good parts I found the overall message a let down and the execution flawed. Not up to the standard of the authors previous work.