Very sad. I remember reading my first Discworld book and wondering what the heck kind of craziness I'd gotten myself involved in...and then I read more. His humor (especially the hilarious footnotes in all his books) got me, as did his social satire. Such a great writer.
His daughter's tweet, from just a few minutes ago, says it best:
"To dad's fans - Cherish what he gave us, every book. They are marvellous gifts of laughter, wisdom and empathy. Through them he is immortal."