The thing about Ian Banks is that his left politics are nowhere near as extreme as fancied.
The big thing, is that sensationalism for its own sake is, remarkably, not forbidden by his political views. The Wasp Factory is not devoid of humanity but it's very like the bait on a hook: We feel something for the characters so that their dire fates are more frightening, more horrifying. When the protagonist of Complicity confesses, then finds he has cancer, he is being condemned for his complicity as thoroughly as any National Review subscriber could wish, while the killer is deprived of the love that was his driving motive. The protagonist of The Bridge even approves the Falklands war, as I recall.
So the notion that the Culture, as a true communist utopia, is the subject of Banks' work strikes me as a fundamental miscalculation in reading his novels. That's why the Culture is almost always off stage, with the bulk of the action taking place in lesser societies. Banks' tendency to sell shock and sensation is the driving force in most of his fiction. He does it with more wit, imagination, style, even depth of feeling than the vast majority of sensation mongers. But that is still his primary stock in trade.
That is why most of his work is essentially set in Ruritania, an imaginary land inhabited by people from other novels. Banks is as good a reader as he is a writer, so this is apparently not so obvious. The proof is that, in fictive universe terms, the inhabitants of the Culture, are not human beings. (Does State of the Art show some connection? Haven't found it yet.) Yet the Culture novels are written as though the characters, by and large, were human. The aliens and the Minds and knife-missiles and drones are rarely main players but wise commentators and plot henchmen instead.
The Algebraist and Matter are showing some signs of growth. In the US, his mainstream novels are even harder to find, though, so it's harder to be sure. As it is, The Bridge is probably his finest novel to date, with very little new thematic material found anywhere else, nor any handled better. The Wasp Factory and Walking on Glass have a lean, tight, pointed impact; Complicity has a topicality forthright politics; The Crow Road has a familial feeling and Scottish background. These seem to stand out more but it's pretty much a matter of taste. If you have read The Bridge, you've gotten, so far, at any rate, the essence of everything Banks has to offer.
When Banks has nerve enough to write science fiction set on Earth, the clash between his formal leftism and the rest of his ideology will force him to find a new voice.