I saved my pennies while sitting in one of these* so I could give myself a gift of back relief, a Life Chair. It costs as much as the Aeron and is at least as comfortable, if not more so. Rather than mounting a dark, mechanistic rig, the Life is visually lighter and seems to yield more comfort for being so. Knoll's extensive line of fabrics can make customization a daunting task, not made easier by their lack of a thorough website. Luckily there's a showroom nearby, as you pretty much have to go through a Knoll dealer to order it, with a process like ordering office furniture. In my case it was worth; I've got one happy bum.
* Note that the Internet includes a metalfoldingchairs.com