Accelerando is an attempt to realistically portray the world as it will exist in the beginning of, during, and after a technological singularity. See my review of The Singularity is Near if you don't know what that is, but the basic idea is that technological progress is accelerating, and in not very long will be so rapid that society and humanity will undergo unimaginable transformations that we can't possibly predict. This book is Stross's attempt to predict them.

I really loved the first third of the book, portraying the life of a neo-entrepreneur who is on the bleeding edge of post-humanism. This portion of the book is jammed with cultural and historical references that very convincingly ground the more fantastic elements and succeed in painting what our future will look like (unless the global energy crisis turns out to be a much bigger problem than most singularitarians predict). I'm very much looking forward to possessing my own meta-cortex, a distributed, external, electronic brain augmentation.

The middle third, when the solar system starts getting dismantled to be turned into computronium, was less engaging. At this point the chief protagonists have uploaded themselves into a life-like simulation, so there's not much in the way of interesting technological speculation -- everything is just simulated, which is only a little better than saying "it's magic."

There is one interesting conversation in this chapter, when several characters discuss whether the singularity had occurred, was occurring, or will occur in the future. One character argues that it happened in the sixties, when DARPA sent the first network packet -- everything after that point was fundamentally unimaginable by that generation. Another character argues that this singularity won't occur until the solar system is made entirely of nanoprocessors configured in a Matrioshka brain configuration. It's a clever reminder that an exponential curve looks the same at every scale, and the definition of any such event is ultimately subjective. (For me, brain uploads are a pretty good indicator that the singularity is now.)

The final third is even farther into the future and concerns the fate of only-posthuman intelligences, who now comprise a tiny minority of intelligence in the solar system. Things by this point look no different than the computer simulation chapters, except that they're happening in real space. Again, kind of disappointing after the impressive beginning.

If you're interested in the singularity, there's lots here to enjoy. But as an overall work of fiction, Accelerando is a bit jumbled and confused, not to mention poky, for my tastes.