The plot is gripping, the characters varied, and the "universe" Iain creates is fascinating, convincing and detailed. If you like sci-fi, then you'll love this.
p.s. Good though this is, I've decided that I've got a bit ingrained with my reading habits. Too many of the books I read in the last year have been by Terry Pratchett or Iain (M.) Banks. If anybody has some suggestions of things I books that might expand my horizons a bit I'd be interested (particularly if you can do it without spoiling the plot and message of whatever book it is). I've just started Brave New World by Aldus Huxley, which is a bit more of a classic and has a much more serious message, but it's still definitely sci-fi.