I guess I lucked out... I bought an Aeron on-line (on-sale) directly from the factory in Medium (because I'm medium). Most comfortable, adjustable chair I've ever had. It has two kinds of castors, for hard or carpeted floors. I have hard, so I ordered those castors. I roll back and forth between my desk and my workbench constantly, fast, and it brings a smile to my face.

Also, this is the only chair I've had whose height adjustment is rock solid. All the others, which were not as comfortable to begin with, started to leak after a few years.