When I took a class at Corning, they used the most comfortable working chairs I've ever used. I'm saving up for one.
The company is
www.biofit.com, and the desk chair model is 4L57-R. (The ones at Corning were the drafting chair height.)
They're kind of pricey ($187 or so) but they're really comfortable and provide excellent back support. Plus, they're not covered in fabric.