I started my own business about 18 months ago (web development) which involves me sitting in my chair for pretty much most of the working day. Did about 4 months in a cheap £35 chair from Viking and then took the plunge with a brand new chrome aeron.
Has to be the absolute best purchase I've ever made (closely followed by the 30" Dell monitor), I can sit in it all day long in absolute comfort (as long as you sit in it properly). As I'm sitting at my PC all day, I would rather spend £800 on a supportive, comfortable chair than £800 on chiropracters fees because I didn't want to spend some money on a decent chair.
There are a lot of alternatives out there, I looked at quite a few, but none of them fit the criteria quite like the Aeron for me. Plus if you buy new you get a 10 year warranty, I had a creak develop in my chair and they came out to fix it twice