My rating: 4 of 5 stars
I like the premise – that the moon explodes and, inevitably, the debris will destroy the earth. Humanity decides to vastly expand the international space station to create the seeds (literally) for survival of most species. As with all Stephenson books the science part is believable and thoroughly worked out, but as with most Stephenson books, the characters are a bit woody and the descriptions a bit long. Better than Anathem, more fanciful than Reamde, but his best books remain Cryptonomicon and the Baroque Trilogy, in my opinion.
That being said, I gave this four stars because, well, there is an element of suspense, and I like the thoroughness of how he works through an idea.
Update 30.6.15: Here is a podcast with Neal Stephenson discussing the book.