[pacaro]

I've used a Stokke/Varier Wing Balans for about 15 years on and off. I find that it allows for a variety of sitting positions.

If you are experiencing knee pain, you may want to adjust so the top of your shin is against the pad, not the knee itself, and keep the majority of your weight through your core and seat.

I started using one when I bruised my coccyx (snowboarding accident) and found it to help with lower back pain YMMV

[edit] I prefer the Wing Balans because it is height adjustable with a gas ram, so it can be used in more positions, with a variety of desk heights. I would often use it very low in more of a kneeling position, or at the upper edge of its height range more like a stool