I had bought a chair similar to the one Jan in VA shows ... only it came with arms that I promptly took off.
Since then I graduated to a Swopper that I bought from an specialized office supply store after trying it for a long weekend. I love it. It helps me keep my posture correct and is adjustable. The only downside is it doesn't have rollers on it, though they could be put on it now [I bought mine years ago.] I note that Sit4less.com has them now.
ali