I have used three different Aeron chairs (I was in the Air Force for six years, and Aerons are used fairly often as a way to burn excess unit funds at the end of the fiscal year). The first was the large size, perfect for me, with all the bells and whistles. The latter two were medium sized and were the base model.
Honestly I've never been that impressed with any of them. The full-Monty model can be adjusted to be quite supportive, and thus is good for sitting for long periods, but even that one is not that comfortable. The base model is really an unexceptional product IMO.
I recently bought a used Herman Miller Equa 2 chair off of Craigslist for $60. Frankly I think it's 85% of the chair the Aeron is, at 40% of the price new (mine is fully equipped, including pneumatic lumbar bolster - I think it sells for $400). Since it doesn't have the name appeal of the Aeron (nothing else does), it, like Steelcases or other quality chairs, is available very cheaply secondhand.