I am currently not quite halfway finished with my third Sketchbook Project. It's a lovely exercise, a bit frustrating, challenging, and super satisfying when you realize how much progress you've made. You sign up, fork over money, they send you a book, and you have until January to get it back to them, filled with whatever; and then it goes on tour.
My first sketchbook was a bit of a disaster. I choose my theme poorly, let myself become completely flabbergasted by the thin paper in the book, and didn't actually finish it until June - a full 6 months after it was due.
My second sketchbook was also a bit of a disaster. I finished it in about two months - a full 10 months before it was due; once again with a poorly chosen theme and execution, and then sat on it for awhile, trying to decide whether I was going to actually submit it, or set it on fire.