Now, $600 + dollars are definitely not what I would normally think about spending on a desk chair, but this one might just be the one to spend it on if you have the money.
It's the new Devon Leather Chair by Restoration Hardware. I tried it out the other day in their San Mateo store, and man oh man, this chair was as comfortable as it comes. Just the right amount of support, good arm rests, and soft movement make this chair a keeper.
It may just be that I'm sick of my hard wooden (makes your butt numb) chair, but this item is definitely going in my wish list.