I have a classic "Stokke" kneeling chair (now apparently called "Variable balans", http://www.varierfurniture.com/en_us/Movement-Chairs) and I like it, but only for temporary use, not as a full-time chair. Two reasons:
First, your back has to be in quite good shape to avoid dropping your posture after an hour or two. If you can't stay in good form, you need to strengthen your back and work up to it.
Second, the pressure on my kneecaps causes a certain amount of knee pain after using it for extended periods.
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
First, your back has to be in quite good shape to avoid dropping your posture after an hour or two. If you can't stay in good form, you need to strengthen your back and work up to it.
Second, the pressure on my kneecaps causes a certain amount of knee pain after using it for extended periods.