I've owned a few ergo chairs and used each for long periods of time. Here's my take as a 6'2", 185lb male:
- Aeron: be wary of considering this as any ergonomic standard. The mesh is great for air and comfortable on your butt, but the back support is weak and it's not very adjustable. You do need to make sure you get the right size (A, B, C). The PostureFit adapter actually hurt my back when I had it dialed in at all. Sold my Aeron after a month.
- Steelcase Leap: This one is also popular among coders and the first version ("v1" on the net) is actually pretty awesome. I've used it days on end, and realy came to appreciate the reclining support and the many adjustments which help keep things fresh. The v2 isn't as adjustable, and the front of the seat pan dug into my thighs so I sent it back after a few weeks. I've owned two of the v1s for about 6 months and spend 10-60 hours/wk in them.
- Soma Ergo. My top pick of the three. These are so obscure that I don't know if you can even find them outside of the Bay Area; I got lucky and found them back when there was a vendor in Palo Alto (they're made to order in Berkeley, I believe). If you go into a store that sells these, they custom fit each element and you choose from various materials. It comes out to about $1000 for a best-materials seat (this includes some fancy cushion material that is the most forgiving of any I've tred after long long coding sessions). I bought mine in 2006 and it remains my favorite chair. Aeron didn't come close. After sitting on the Leaps, my body always feels relief coming back to this one.
If you're on a budget, the fully-adjustable Patriot that they sell at Office Depot is also pretty good: it's only $200 and I'd take it over an Aeron without hesitation. I used these for about 8 years. Main limitations are that the seat bottom will hurt after long hours and you'll probably want supplemental back support if you're prone to back issues.