I was invited to join a Round Robin based out of Hawaii a couple of years ago. The exchange began July 2010 and ended in July 2012.
We were to pick a theme for our book and 21 other participants would contribute to it. In exchange I would contribute to all their books. All my contributions can be seen here.
My theme was simply "Circles". Some people had rather complex themes and very detailed instructions. I found some of them very difficult to work with, some quite a challenge and others a delight.