They are standalones, although some characters may re-appear. Anyway, one of my favourites is "Use of Weapons", which is one of those books that you reach the end and then want to start reading all over again with the knowledge of the ending, to start appreciating the details that now make sense.
I am a big fan of Banks in general, also of his mainstream novels ("Complicity" and "Crow Road" are probably my favourites). And I felt literally sick for a few days after reading his debut "Wasp Factory".