One of those books that picks you up and takes you on a journey. I was ambivalent about it at first, but I quickly became enthusiastic. Reynolds' style allows for an ease of exposition: in a couple of sentences, he can give you an idea of the nature of several different civilizations without going too in depth. This skill allows him to construct the epic scope required for a space opera of this nature.
And epic it is indeed. Spanning millions of years, House of Suns deals with space travel in a refreshing light. Most science fiction revolves around a method of superluminal travel. This is necessary in order to tell a story in a comprehensible length of time (it's not necessary to tell a story, as many authors demonstrate). Reynolds combines a number of hard SF concepts (cloning, sleeper travel/stasis, memory alteration, etc.) while avoiding the whiz-bang action of superluminal travel. This requires a great deal of relativistic action (literally), and it's at this point that the science part of the book may be difficult for some readers to comprehend. Chances are if you enjoy reading hard SF, however, it's because you enjoy things like relativity!
For me, some of the scariest concepts involve memory and the nature of self. The main characters are two of one thousand clones of a single human woman who lived (in the time the main story is set) approximately six million years ago in an expansive human empire. Yet these clones have been genetically tweaked to have different personalities and characteristics (to the point of having different genders). They are explorers, reunited after circumnavigating the galaxy in order to share the knowledge they've collected. It's an interesting concept. The idea that you can map someone's mind, download his or her memories and duplicate the consciousness--the self--is scary. It's been discussed over and over in SF, of course (I particularly enjoyed Robert J. Sawyer). Reynolds wields this concept masterfully, because many of the main characters are actually radically divergent copies of one original person. It just gets weirder from there.
I figured out who the traitor was before the traitor was unmasked. That doesn't stop the enjoyment of the plot. Unfortunately, the last act of the book did let me down somewhat. I am hoping this is the first in a new series, because otherwise the ending is postmodernly ambiguous. It's hopeful but nihilistic at the same time--working in comsological time can do that to humans, as the main characters observe. Our consciousnesses are not built to function for millions of consecutive years (which is why the main characters have only lived for several thousand subjective years, biologically possible thanks to technology, while entering "abeyance" during extended space travel). The implications of events in the book mean that even if you take the ending to mean that our protagonists have ultimately succeeded, it's possible that the people they were trying to save no longer even exist.
The last act of the book does slow down and get more predictable, although one part did give me a delicious sensation at the end, when we learn the true reason for the Absence. It's clever.
Well worth the read if you're into hard SF!