The brief version: Fuckin' awesome.
The more characteristically verbose version: Greg Egan's Diaspora is the best hard sf novel I've read in a very long time. It's not exactly light reading - but most hard sf isn't meant to be.
Most of the POV characters are entirely made of sentient software. There are three primary strains of human life in Egan's future history: The ﬂeshers (biological humans), the gleisners (software minds inhabiting robot bodies) and the citizens (software minds, and digitized humans, living entirely inside a supercomputer complex). The lead character is a citizen, and we literally follow vis life from conception till … well, for lack of a better term, completion.
"Vis"? Yeah, most citizens "born" after the Introdus (the historical event where much of humanity decided to be digitized and become citizens) don't have a gender identity, and use the neutral pronouns ve/vis/ver. This might initially seem jarring from a purely linguistic point of view (since those are not actual English pronouns), but I quickly got used to it - particularly because it makes sense; why would late-generation software entities feel in any way connected to a social role that is so based on a biological concept that no longer has any real meaning for them? There are a few citizen-"born" citizens (including one main character) who do adopt a gender identity - their "mind seeds" are based on algorithmically generated variations of what was originally human minds, after all - but in general, citizens are asexual and genderless.
I had prepared myself for a book that would concern itself with conflicts between these multiple ways of being human (eg. a "space opera"-ish story similar to Bruce Sterling's Shaper/Mechanist works or Alastair Reynolds' Revelation Space cycle), but there was actually remarkably little of that. The gleisners and citizens mostly exist in a state of semi-friendly nonviolent competition, and both are highly protective of the ﬂeshers (who, however, tend to be suspicious of nonbiologicals). It's strongly implied that there has been violent conflicts between the ﬂeshers and the nonbiological human life, but by the time of the story, peace has reigned for decades. There is, in fact, only one violent scene in the entire story.
The central crisis of the story isn't warfare or a villainous plot: It's an astrophysical disaster, calling for scientists and explorers and diplomats, not warriors and generals. It's about humanity, now divided into multiple distinct posthumanities, trying to avoid extinction in the very long term. It's about the grim realization that a mistaken view of theoretical physics led to an extinction event that could have been predicted (and possibly avoided) using better science.
Consequently, a lot of the "action" isn't of the spaceship shoot laser, BOOM! variety (with apologies to the Nerd), but about people exploring, making scientific discoveries and trying to make sense of those. Consequently, there's a lot of strange multidimensional particle physics speculation and "techtalk" (as is common in the genre) - which I enjoyed (even though I admittedly found the ﬁctional wormhole-based physics paradigm of "Kozuch theory" weird). There's also personal tragedy, love, hate, regret, failure, victory and joy. Most of which happens to be the tragedies, love affairs, failures and victories of people made entirely out of software.
It is, in other words, an excellent hard science ﬁction book. I found myself almost unable to put it down after I started reading it. Highly, highly recommended.