I'd read Permutation City previously, despite the fact that Egan wrote this book first. They cover similar ideas, although PC was more convoluted, as Diaspora had nicked all the low-hanging fruit. I find his novels horribly scientifically plausible - a vision of the future more alien, yet somehow more believable than the extrapolation of, say Accelerando. The background to this particular story is a bleak utopia, even worse than Banks's Culture. What do you do in a world where you can do everything?
Specifically, the main characters of this book are conscious pieces of software running on hugely powerful computers in the distant future. They can do anything they want in the simulated world. Then life in the (real) flesh is wiped out, and the characters look for a way to avoid being hit by similar catastrophes. The plot is rather uninspiring, but provides a backdrop against which Egan explores maths and (well-researched!) physics, and the consequences of digital consciousness.
The characters are surprisingly human, for disembodied software. The politics and emotions are all still there, which I guess is his concession to avoiding writing a book so alien as to be almost meaningless. There are aliens who are 'too alien to comprehend', but I didn't find that part particularly convincing. On the other hand, looking at some of the main characters, it's entirely conceivable that intelligent programs might act a bit dumbly from time to time!
As usual, it feels like Egan's snuck inside my head, taken my thoughts, fleshed them out, explained them better, made them into a story, extrapolated logically, and come up with a depressing conclusion. In this case, more literally than usual, because I found the ending itself uninteresting and uninspiring. On the other hand, perhaps this is the message of the book: It's about the journey, not the destination.