This is what I have at work, not an ultra pricey model, but never had back problems with it and right now I work 10-12 hour days, 5 days a week. The biggest downfall is that the arm height is not as adjustable as I would like. I wished the arms went a tad lower since my desk is low. Otherwise, if I start feeling any back stiffness it is easy to adjust many different points of the chair. Good lumbar support and it has adjustment for the back and seat angle. Had it for 4 years and still looks brand new. I do the ergonomics at my company, and these chairs seem to be very well received, and nobody needs to purchase additional lumbar supports for them.
http://www.officedepot.com/a/products/395007/Office-Star-ProGrid-High-Back-Fabric/In the world of ergonomics, things like those wacky kneeling chairs really only force you into a better posture. The trick is to just find a chair that is both comfortable and it feels natural to sit properly. A $5000 ergonomic chair means nothing if you don't feel comfortable sitting properly, and to be honest, I have found that desk and monitor height can be far more important of a factor than the chair itself.