I was just about to buy the Aeron chair until I read this:
"Having sat in an Aeron for several years, the Humanscale Freedom is a different animal altogether. No more legs falling asleep. No more silly knobs. No more aching back. The Freedom is truly free of any impedence to the enjoyment of taking a seat."
And a few more reviews that said even though the Aeron chair was good, the Freedom was tons better. I know most of the people who dont like the Aeron chair probably just had bad expiriences with it, but I havent found anyone who doesnt like the Freedom chair. Thats how I chose between the two, but im sure the aeron is worth its $700.
EDIT: My freedom will be here sometime around the 16th-25th, I will post about it then and see if it was worth the money.