Our main character is Horza, a Changer (given a few hours - or days, depends on the change - he can change his appearance to that of another humanoid, complete with fingerprints). The book opens with him chained up in a room attached to the toilets of the banquet hall above him: his execution is to be by slow drowning in the shit and piss of the partyers above him. Banks spends several pages on this, including lots of unpleasant detail. All of which reminded me that Iain Banks famous first book, The Wasp Factory, was notorious for its violence and grotesquery (Wikipedia says 'The book sold well, but was greeted with a mixture of acclaim and controversy, due to its gruesome depiction of violence. The Irish Times called it "a work of unparalleled depravity."') I hoped that this lovely introduction had him getting it out of his system, but it was not to be: about a third of the way through the book he introduces us to a cult that eats only refuse. Except for their extremely obese prophet, who eats people while they're still alive (that section gave us 40 pages of the grotesque without advancing the plot at all).
This combined with the oft-repeated out-of-the-frying-pan-into-the-fire events that leave Horza constantly in danger and travelling all over the galaxy while barely advancing what I had thought was the main plot about the war between "The Culture" (an "anarchic utopia" of Humans and A.I.) and the Idirans (religious fanatic aliens that Horza has chosen to side with "because at least they're on the side of life" - he's not a fan of A.I.) nearly put me off the book - but his writing was otherwise good enough and mostly entertaining enough to keep me going. Still, the last 50 pages of this 450 page monstrosity (what makes it a monstrosity is that it should have been 250 pages) were a horrible slog - I was just sick of it by the end.
Banks' The Player of Games and Use of Weapons (the next two books in "The Culture" series) are highly recommended, both by the SF community in general and by a friend who didn't think much of Consider Phlebas either - he thought it was too long, and too grotesque. Given the similarity of his point of view, I should probably move on to the other two titles. But I've been so put off by this one it may be a while.