I have always really enjoyed this one. That is despite it being a very sad book and it having a whole section that I feel should just have been dropped.
Firstly, the sadness. This one is about grief and the despair that can stem from grief. It is absolutely unflinching in this regard. It does not tritely wrap up these stories with grieving people moving on and finding a new purpose etc. It takes the despair them all the way to a very logical conclusion. A brave choice in regards to the 'saleability' of the book. People don't like to be bummed by their entertainment.
Secondly, the unnecessary (in my opinion). There is a whole subplot around an, admittedly neat, alien biome that is loosely tied to main plot. I say loosely as it is supposed to be a timer on revealing the central conspiracy but is shown to be absolutely redundant to the resolution of the story in the epilogues. I really do feel Banks wanted to describe the biome and so just attached it tenuously to the main story to give it a reason to be there. Caveat, there may be something really clever going on with that sub-plot that I missed (a familiar feeling for me with a Culture novel) but I can only record my impressions in these reviews.
Still and all, this is a Culture novel. It's also about the limits of Utopia and Banks is certainly not afraid to have his utopian sociery do a big oops (which is the raison d'etre of this book). However, its the descriptions of Culture living and speculation about other ways of doing things that holds the attention. One thing I picked up from this one is how eerily polite everyone is in Culture books, even when the mass murder of billions is being discussed and enacted.
Been doing some other things as well so this took longer than it normally would for me to get through. No reflection on the quality though. A definite recommend.