I recently bought a Steelcase Leap v2 and it's the greatest chair I've used. Purchased from ebay used and reupholstered, but a week later they were selling new ones for £50 more (typical :().
If you spend a lot of time in it, it's worth getting a chair that you can adjust to match your body and environment. I've used a good number of random ones through work and none have been as adjustable or as comfy.
http://www.steelcase.co.uk/en/produc.../overview.aspx