I still like the Aerons. You can get a cheap knockoff on Amazon for $150-200, or a used one off Craigslist. Whatever one you end up going with, I'd recommend trying it out in person first.
1) They're overpriced. You can get an equivalently ergonomic chair for $100.
2) The Aeron chair was never actually designed for ergonomics. It was designed for style. It's a really common misconception that the Aeron chair was designed to be perfectly ergonomic. [1]
It goes without saying that the most ergonomic chair is going to be one which you modify (by adding various supports / strap-ons) to fit your body.
[1] http://www.businessweek.com/magazine/content/10_19/b41770712....
Just make sure you get long enough cables if your computer is under your desk.