Not just great science fiction (space opera is new term) but he also writes regular fiction, mostly set in Scotland. These books are equally great. To distinguish, the Iain M. Banks books are sci-fi, the Iain Banks books (no M) are fiction. There are a few crossover novels (Transition in particular).
Definitely one of the best writers working today (not just sci-fi).
Wildly imaginative, deep (def. in terms of detail and description, frequently in other ways, too), intricate works. Expansive worlds to explore. Cons: Length might be an issue for some. Also, the amazing build-up and tension in some of his books leads to a bit of a let-down at the end (either because you want more, or because the resolution isn't / can't be adequate).
One of the best new Space Opera writers, part of a great group of BRitish writers revitalizing this genre. Cons: Some dialogue is a bit wooden, not much in the way of cons if you like this type of fiction.