Well, it looks more like a stool to me. I haven't been able to quit thinking about it since sitting on one last week while looking for a office chair for my daughter.
The Swopper is a German design, with a convex seat. It moves somewhat like a pogo stick: up and down, side to side, etc. It does away with static posture since it keeps you in motion a lot. The seat promotes proper posture. Because you are moving on it alot, it helps to increase core and back strength.
I was amazed how comfortable I was on it, despite the notion that it looks silly. I would love to hear from anyone who has actually used one for more than a couple of minutes.