We have a Knoll Barcelona, and got a Aarnio ball for our son for his 10th birthday. He had sat in one at a design exhibition and loved it. I find it disconcerting, as it deadens sound in the room too much.
We also have some armless MR Lounge chairs, also made by Knoll, picked up at a garage sale; the leather straps had suffered, so I made up some new straps, but they're OK for the most part.