I've tried sitting on the Salli Twin ( http://salli.com/eu/Frontpage ) for a few minutes and am seriously contemplating buying one soon. It made me sit proper and comfortable from get-go. On normal (even expensive) office chairs I tend to numb my lowerback/legs at some point of the day. Bonus: It's also been built with men's genitals in mind ( http://www.youtube.com/watch?v=_HIt3D7Ivfc ).
These originate from Rautalampi, Finland. Just over 40 miles from where I live.
I'd also recommend trying before buying.