I have a Herman Miller Aeron that has served me well for the last 15 years. I've sat in that chair at least 10-12 hours a day, 5 days per week for most of the time I've owned it. When I spent a 2 year stint working for Google in a cube farm, I really missed this chair.

What I like about the Aeron in addition to the ergonomics, is that it is mesh, so it tends to allow me to stay cooler than most chairs.