I slipped 2 disks in my back at the begining of the year and went out and tried many chairs including the hermann miller range, I eventually settled on the RH Logic 400 with the XL base.
I sit on the chair for 9 + hours and no longer suffer from fatigue come the end of the day. Takes a while to get the chair set right for you though.
If you are spending that amount of money on a chair goto a good shop to try as many out as possible and be fitted for the chair, don't just go to an office furniture store that holds a particular range as the salesmen are just interested in selling you the one that returns them the most profit, speaking from experience.
regards,
Matt