A new Aeron comes with 12 years warranty but the warranty cannot be transferred (it's great if you can buy a $300 one out of a failed startup but you won't have any warranty). I've got mine since 4 years : I use it daily and for extended period of time and it looks brand new. No wear whatsoever. Should it break in more than 8 years and hence be out of warranty, I'd buy a new one in a heartbeat.
Also there are many pieces you can buy on eBay for your Aeron: for example I added a coat-hanger on mine. But you can also buy replacement / refurbished part.
I don't know what the person saying you cannot configure a Aeron is smoking but I'd like to have some. I took me 30 minutes to configure everything the day I received it, following the instruction in the manual and the setup is perfect and the setup never ever moved. Basically if you put your hands behind your head with your feet slightly of the ground (say one inch above the ground), the chair must lean back. If you then put your hands back on your keyboard, the chair must lean forward. This is pure perfection IMHO.
You can configure how far the chair leans back, how fast it leans back and how easily it leans back. Pretty sweet.
Anyway you cannot tell if you like a chair or not until you've sweat enough in the chair: to me the pellicle mesh on the Aeron is the single best thing ever invented to prevent excessive sweating and that's why I could never ever go back working on a chair which doesn't have the same kind of mesh (both for the back and for the butt: some chairs have a mesh but only for the back, that's not good enough). To me the ultimate chair fail is saying you have a comfortable chair and then putting a towel on the chair: sad.
My sister-in-law had an Aeron at her job and told me she didn't like it. it's a huge company so I told her to call someone to help her setup the Aeron correctly... She then bought one for her home too ; )
Most criticism I read about the Aeron are from people who don't know how to set it up correctly: you cannot just sit in the Aeron of someone who doesn't have the same weight as you and make an opinion about the chair without first configuring it to your size/weight. For example if you sit in the Aeron of someone weighting more than you, you'll have the impression that the chair pushes to heavily in your back and that's not comfortable.
Truth is: many people know how to setup their Aeron correctly and there are more people who tried an Aeron correctly set up which like the Aeron a lot than people who do not like it.
I don't have much to say about the Steelcase Leap: I keep hearing good things about it but without a pellicle mesh "for the butt", this chair simply ain't for me
Hope you find something that you like, good luck and let us know what you eventually settled for!