I received my Herman Miller Aeron Chair a few days ago. As promised, here is a short review.
I ordered the most basic model (i.e. cheapest) which was still $600. The basic Herman Miller Aeron Chair comes with adjustable seat height and back rest (a knob adjusts the amount of tension required to lean back in the chair). The more expensive Aeron Chairs also have adjustable armrest, arms that swing in and out so you don't always have to use them, and I believe there is also some sort of adjustable lumbar support. Unfortunately, the snazzier Aeron chairs also cost a couple hundred dollars more. Because a good ergonomic chair should have adjustable armrests, I almost wish I had sprung for the more expensive chair, but this model feels good and I am pleased with it.
Because I ordered over the Internet, I did not need to pay tax. I ordered the Aeron from Sit 4 Less (links at the bottom). Shipping was free and took about a week to arrive. Assembly was extremely simple. The chair arrived in two halves (bottom and back rest) which were easily connected with the accompanying Allen wrench and four bolts. Even with the help of my cat (who has a penchant for laying on directions), it only took a few minutes to put my chair together.
The Aeron Chair itself is quite sturdy; it has a solid frame that could take a beating and keep on, er, seating. I remember being surprised at how heavy the box was on the porch (of course, most things seem heavy when being dragged up a flight of stairs).
The seat and back are made of a sort of dense mesh that is designed to give good support yet remain cool unlike those leather chairs. The basic casters work fine on my short carpet (Herman Miller has some alternate casters for deep carpet which cost more).
My old chair tended to suck me in, and it was big enough (too big) for me to fold my legs under me or throw them up on the desk, not really the ergonomic thing to do. It was really too large for me. The Aeron Chair is not at all restraining or inflexible, yet it encourages my body to maintain better posture because it fits better.
In conclusion, I believe my Herman Miller Aeron Chair is a solid piece of equipment that will last me for years to come.