Please Help Me Choose a Worthy Computer Desk Chair

I am in the market for a good computer desk chair. I mean one like Chris Pirillo has in his office -- the Grahl Synchron 8, which apparently is not sold publicly anymore. I could do without that crazy price, though. ;)

I am looking for full body support to help keep my back as pain free as possible. My sciatic nerve is really irritating me. I am willing to spend a pretty fair amount for a chair that will keep me comfortable and pain free.

Some others I am considering are:

The Kneelsit

The Mirra

The Aeron

Any recommendations?