I've recently had the pleasure of testing one of those fancy ergonomic chairs for a few weeks, when my back and neck were torturing me. My chair of choice was the Liberty chair from Humanscale, the version with the headrest.
I immediately noticed a difference between that and the chair I had been using--the Liberty's seat and back cushioning was thicker, and much firmer, than most business seat chair cushioning.This firmness the Liberty chair seems to have in common with other ergonomic seating.
The ability to occasionally recline is there, and is welcome. (Another standard of ergonomic seating, in allowing you to comfortably sit in multiple positions during the course of the day.) You can adjust the chair in a variety of ways, in terms of height, especially. However, the adjustment that I really like is the ability to change the height of the arms; often, they are a bit too high and rigid (which can cause you to tense your shoulder and neck muscles), and don't allow you to type in a number of position, such as typing with your keyboard on your lap--particularly important if your desk doesn't have a keyboard tray (and is handy even if you do have that amenity).
The only real problem with the chair, other than price, is the headrest (although you can opt for the Liberty chair without the headrest). For some reason, it slopes forward (huh?), which made my neck muscles tense when I did a back stretch, or simply decided to lean all the way back in the chair. However, my back and neck pain in general seems to have greatly diminished after just a few days of sitting in Liberty.
I've also tried (for briefer periods of time, regrettably) a few of the chairs and other seating from Herman Miller. [Disclaimer: I may have a financial relationship with Herman Miller in the future.] I tried the Aeron, the Mirra, and the Cella chairs (the "lower-end" Cella is more comfortable than it appears). All these chairs are highly adjustable; you can even adjust the amount of pressure that's applied to your back, as well as chair and arm height. (Yay!) You can also adjust the "springiness" of the chair (how quickly the chair snaps back after you recover from a reclining position). These chairs are lighter than the Liberty (and the Mirra is available in a couple of different coverings) and easier to scoot across the floor, as many Herman Miller chairs are (even the non-ergonomic ones, such as the various stackables). Also, they have the good balance of firmness that most business chairs lack. (Even Herman Miller's Goetz sofa is firm and comfortable, and easy to get up from, unlike many sofas.)
Now, I realize that to improve and maintain back health, your work chair is only one component of the equation--you need exercise (and perhaps even the occasional massage) and to get away from your desk during the day, periodically. However, as many people spend long periods of time at work in front of a computer, the chair on which you are parked can play a major in improving and maintaining your health, and can help you become motivated to move more, if only because you may be in less pain when you do move!
I know that lessening of pain is an issue for me, and for anyone who has returned to work after an injury, and using an "ergonomic" chair can mean a noticeable improvement with regard to pain.
Regarding price (which is where the real pain crops up), scour the Internet for price comparisons; they run a few hundred dollars, but are built to last. A good ergonomic chair is a true investment in your well-being.