I dunno, but I do know that even the first day I started this job I HATED this chair and had pretty big problems from the start. The chair ends right at my shoulderblades and just digs right into them. Whereas my slightly better chair at home is... slightly better. And my uncle has some fancy high-backed executive computer chair and I can sit at his house and screw around on his computer for hours... but he's rich and I would bet his chair cost a lot.
I think, when you're right on the cusp, a decent chair can matter a lot. Whether ergonomic chairs specifically actually work, well... I don't buy into marketing hype, that's why I'm trying to get real opinions from real people, though I don't seem to run into anyone who ever actually tried one.
The main points have been touched upon and these are simple and cheap.
The problem with chairs are that they are often bought with no thought in mind of the desk that they are being used with.
Arm rests, as any physiotherapist will tell you (like mine did) will tell you that these are a major cause of back problems. You should support your upper body by the spine and not in your arms by resting them on the arm rest.
When your sat in your desk chair, if it is adjusted to your needs correctly so that you are sat with feet flat on the floor and the desk is unconfortable, then the issue is with your desk and not the chair.
The main point I am making is that education is all it takes and if your back problems you feel are from your chair, you will be sorly mistaken, speak to a physiotherapist about it and you will find out what I am saying is spot on.
For me, this is a open and shut case, if you read here : http://www.backpainexpert.co.uk/Physiotherapy.html : http://www.sydneyphysiotherapist.com.au/back-physiotherapy.html : http://physio-net.com/reference/commonconditions/backpain.htm are amongst the few top reults I got for physiotherapy for back problems and http://www.spine-health.com/wellness/ergonomics/good-posture-helps-reduce-back-pain : http://news.bbc.co.uk/2/hi/health/7568948.stm : http://www.cure-back-pain.org/bad-posture-back-pain.html were found looking under posture for back problems
You have a whole resource at your finger tips and the answer is in one the links and not a new desk chair.