Hi
We had one of these for working on the PC at home as Husband had a lot of back problems. He found it quite good - I have bad knees and suffered really badly when using it, but that could have been because I couldn't be bothered to alter the height and angle everytime I used the PC after him. (I'm pretty sure it's not supposed to hurt your knees - so it was me not the stool!)
We also used them on my pedicure course with LCN, they were adapted to have an extra foot rest between your knees for the clients foot - I liked the working position, but just couldn't handle the pressure on my knees. One of the other girls who also suffered from back problems said she hated it as bending over made her back ache, but I think with time you would find the best position to sit in.
They do keep your back straight and you don't notice you're not leaning against the back of a chair because you feel really supported.
We got our's really cheaply from Staples (the office suppliers) they had a chair special on and we got it for about £45