For those that don't remember, the ultimate reveal at the end of the book, literally the last paragraph, is that for the vast majority of the book, although you think you have been following the exploits and thoughts of Zakalwe, you have in fact been following the emotional journey of his cousin, half-brother and long term nemesis, Elethiomel, who stole his name.
But it just doesn't stack up.
From flashbacks and through the narrative of the last parapgraph, Elethiomel can be best described as cocky, borderline incestuous, power hungry, conniving, sadistic and psychopathic/sociopathic (he does, after all, skin one man alive and then make a chair out of his sort-of-sister, who he has shagged)...and yet, by reading through the life of the fake Zakalwe we are supposed to believe that that is the same person who goes on to find sex a 'violent' congress, takes prisoners and ends up feeling sorry for them, tries to give it all up and take up poetry, happily does the bidding of the Culture, and although still sociopathic and conniving, is constantly troubled by the nature of good and evil, and his role within it. Essentially, he is painted as a good natured sort who just happens to kill for a living and sometimes struggles with that dichotomy (like umpteen other spys in umpteen other books before and after him)
And that just doesn't tally in any way shape or form. And in structuring the book so that the final reveal is pretty much the final thing you read, Banks also cheats us of direct contact with what is in fact the most important part in the book, and the most important thing in the main protagonists life, that being the killing of his half-sister, turning her into a chair, and then the near miraculous conversion from someone who could kill his half-sister and turn her into a chair to an inward looking brooding type constantly wondering about his place in the universe. Without describing that process, the book has little to no weight in my opinion.
Finally, and this is technical point, I'm sure there are several references made to the mark close to the heart of the bone fragment from his sister. I assume that if I read it back, Banks will have cleverly made it look during the flashbacks that it was Zakalwe that took the bone fragment when in fact it could just as easily be read that it was Elethiomel...but it still seems odd, although it would constitute an almighty continuity error, so I doubt Banks would have been caught out by that.