"About 80% of a great book"
First, the good part. Reynolds has a rare gift for weaving hard science into his plot. The matter of fact limitations of physics and effects of near light speed travel make this novel stand out from a host of lazy sci-fi. The author is undeniably adroit at imagining and describing alien and future worlds and this makes for a compelling and at times irresistible narrative.
However...the science and alternate worlds are by far the best part of the book. The characters inhabiting these worlds are entirely flat and opaque, and I do not mean unlikable. Based on previous reviews I expected the characters might be harsh or unsympathetic, but instead I have almost no idea what their motivations might be. They have little internal narrative, and character development is strictly one-dimensional, A to B. They are alternately self-righteous and petty, occasionally stopping in the middle of the most intense danger to be catty to each other. Whenever they begin talking the story comes to a screeching halt. The dialogue is stilted and odd, and sounds nothing like actual human conversation.
With all of this being said, if you can overlook the paper cut-out characters and get past the parts where they talk, there is a fascinating, magnetic story here. I do hope Reynolds continues this universe in the future, with better characters.