A would-be Foundation or Dune-style epic. The style is tedious and wooden. The story is not particularly imaginative though it has a number of pleasant conceits, ranging from the Canterbury Tales format of the first volume, to the various obsessive exploitations of Keats themes in the second, the zen koans of an AI nammed Ummon, and a pastiche of a famous scene from Romeo and Juliet. The author appears to be a widely-read magpie - occasionally a confused one, mistaking Brahe for Kepler. If his prose style were bearable, it could be quite entertaining.
While I prefer writers with a good style, such as Bradbury or Kim Stanley Robinson, or those who aspire to one, like Dick and Bester, still I'm perfectly happy with Herbert's Dune and Card's Ender series, which are adequately executed and reflect the authors' real commitments to their constructed worlds. In other words, as someone once said (Shaw?) "fortunately, I am a man of low tastes". But not low enough for this. The world of the book is unconvincing, and seems to be based on broad familiarity with the themes of science fiction rather than any internal vision, salted lightly with uninspired sex and violence (not always easily distinguishable) when all invention flags. Even the author seems to have succumbed to the tedium well before the end.
Toward the end of volume 2 Simmons repeatedly recapitulates recent events (from the same volume) in the apparent conviction that his readership is too cretinous to follow the plot. In this, I assume he is mistaken.
The book seems to be immensely popular. It won the Hugo Award, which is a "fan's award", as opposed to the Nebula Award, given by professionals. The Hugo went to Harry Potter this year, which in itself speaks volumes. On the other hand, when the Hugo ballots are counted, they have more submissions for the tv/movie award than for the novel award, which suggests that the generic fans' notion of SF is fundamentally non-literate.