I've got an Aeron at home, and a Mirra (http://www.hermanmiller.com/mirra/) at work. Both are made by Herman Miller. The Mirra is a bit cheaper and doesn't look as swish, but I find it about 50% more comfortable than the Aeron (to the point that I'm thinking about replacing the Aeron).

And both are many, many times more comfortable than anything else I've used.