My Experience with the Adaptic Xtreme
At first, this kind of seated posture is rather awkward, but you'll quickly get to appreciate it. The seat is incredibly flexible and tilts way off the axis. My spine, until recently used to a stiff posture, is doing much better now. For work I find it best to tilt forward, and to relax I lean into the backrest. You can occasionally tilt to the side to keep yourself from going stiff - it's kinda like sitting in a boat.
Like I'd been warned, the first few days the lower back muscles can get a bit sore. That's just the result of them having to work for a living now, where on other chairs they don't. But unlike other chairs, you can get up from a long work shift at the computer without a stiff back. You feel like you've just done some stretching. The difference is really noticeable. I have no idea how I could've worked on a normal office chair up till now. :)
True, these chairs don't come cheap, but a healthy back is definitely worth it.