(I'm not a native English speaker, so please excuse any grammatical errors, awkward expressions, clumsiness etc.)
This is an excellent, suspenseful and gripping novel, a good mix of crime, SF and alternative history. In essence, it is about the uncovering of an evil conspiracy and the attempt to prevent it from happening.
The first part, the detective work, is excellently written and mixes a hi-tech, futuristic 'present' with a 'past' on a version of Earth similar to ours. Suspense is great, and I couldn't wait for the protagonists to finally discover what the conspiracy was all about. It is a great mix of the classical detective novel with science-fiction. It comprises about four fifths of the whole book.
The second (much shorter) part, in which the protagonists try to thwart the evildoer's plans, is a little bit of a letdown, and does not nearly manage to convey the same nail-biting suspense as the first part. This is due mainly to the fact that we never really get to know the 'evil guys', and although their aims are presented, their motivation is not made very clear. Also, I would have preferred the second part simply not to be so short and contain more plot elements and twists.
The characters are - as seems to be typical from this author - believable and lifelike, but even the 'good' ones are often not entirely likable. That's a plus in my book, it keeps them interesting. Dialogue is very good, witty and sarcastic at times (his sense of humour and how it shines through in his books is one great strength of A. Reynolds IMO), and contributes greatly in characterising the protagonists.
The only problem is that in this book, we don't get any interactions with, much less characterisation of, the 'evil' characters. In fact, we learn almost nothing about them, who they are, and what motivates them. For me, that's the biggest drawback of this book. I hope there will be a sequel in which we get to see further events more from the conspirators' perspective...
For those who have read all or some of the author's books dealing with the 'Revelation Space' universe, this one is very different in style, setting and scope - it is not nearly as epic in scale as the 'Inhibitor Trilogy' (Revelation Space, Redemption Ark and Absolution Gap). It's just as good, but different, so be forewarned :-)
This book was a hugely enjoyable, entertaining read, despite its shortcomings, and thus I rate it 4/5.