I can concur the Office Star ($500) chair is very comfortable. But if you looking for one a bit cheaper (and I can conclude its amazing, but I have had it for over a year and sitting it in now), check out this one.
http://www.staples.com/Staples-Culley-Luxura-Managers-Chairs/product_SS1009288?cmArea=SC2:CG32:DP2155:CL142570
I sit in this chair during the week each day about 3 hours and on weekends sometimes up to five hours straight. My back never hurts and is comfortable to me.
Newegg rocks!!!