A.They all cost a fortune and are uncomfortable as hell to sit on. I remain fascinated by people who buy chairs by reputation instead of comfort. Nutty. My Gymnic exercise ball has been my working desk chair of choice since the early 90s: I have yet to meet its match. Amazingly enough, it still costs only $25!
Using one of these balls for sitting is a whole different ballgame. Ahem. They require "active sitting," that is, constant, unconscious movements of the back, torso, and legs to stay comfortably atop them; this is the best possible remedy for back pain and disc problems, as well as being an excellent prophylactic device against same. In addition, venous stasis can't happen, since you are always moving just a bit. And, it's tremendous fun to just bounce up and down while you work. People will think you're nuts: I know several attorneys who've tried and loved my chair but haven't put one in their offices because of its visual frivolity. Too bad!
These balls were originally developed and perfected in Switzerland, then used for decades in physical therapy all over Europe before making their way to the U.S. about 15 years ago. Even so, they're still rather an underground phenomenon here.
Bonus - I took my extra chair out of my office and substituted a second ball-chair: instantly, the number of people stopping by to shmooze and hang out dwindled to nearly 0! Take it from this medical doctor, this is the way to go. Helpful sizing hint: most adults will find the 75 cm size perfect. And, it comes in a heart-stoppingly bright yellow: who could resist?