I've only read one of his non-SF books (they're published as Iain Banks, no middle initial), but I love the Culture novels immensely. Many of them tend toward the grim, but I think that's because they're set against the background of a humanist-utopian setting that would be pretty boring to write much about directly.
I think what I love most about his writing is the sense of things that are vast and wonderful but still have room for the very small and human.
If you haven't read him, do. If you have, go say farewell in the guestbook.