Right now I'm dealing with a wooden chair, like the kind you'd find at a kitchen table. I've found that to be better than anything at staples/office depot for my productivity.
Just make sure you get long enough cables if your computer is under your desk.
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....
I've purchased <$120 chairs for a long time from the usual suspects, Office Depot, Staples, etc. and the padding in the seat just goes to shit after a year. As if I'm sitting on wood, resulting in lower-back-pains galore.
Today, I'm using a Humanscale Freedom with headrest that I got off craigslist used, but reupholstered, for $400. It's one of the best investments I've ever made. I sit in this chair up to 10 hours a day, and not the slightest bit of back pain.
Can't recommend it enough, so my advice would be to seriously consider stretching your budget ever so slightly. Your future 70-year-old self will thank you (and me if you remember this recommendation!).
A good chair is not a replacement for physical activity though.
The same goes for mattresses. You spend 1/3 of your entire life on these things. You should be spending top dollar for the best mattress you can find on the planet.
Get something <$100 that doesn't have armrests and doesn't lean back (or else it locks such a function solidly). It will force you to sit correctly.