The Aeron chairs are definitely worth it, although I would expect you'll see a lot of disagreement here because the price is so steep. Certain people are going to slam the chair on price alone. Having used one at home for many years now, I'd buy another one in a second. When I shipped my Aeron out to get resurfaced (or whatever you want to call it, they put new mesh on), I used a temporary chair for a few days, not only did my back feel like shit, it scuffed up my hardwood floor in only a few days time.
The true testament to me, is that I can sit in my Aeron for 10 hours and never feel any sort of fatigue.
Edit: I'm also going to add that part of my opinion hinges on the fact that I'm 6'6. Whatever height the 'average' chair is made for, if you are that height, the cheaper chairs probably work for you a lot better than they do for me. However, a company like Herman Miller allows me to buy a chair for my height, which probably accounts for 80% of the comfort.