Kneeling chair is great, especially if you have a small body frame. I bought them and used for over 2 years around 1997. They are great alternative to normal office chair.
Kneeling chair may not be a 100% replacement of office chair. because For example i hop around chair with different positions often, sometimes sitting with chest on back rest, sometimes squat on chair, sometimes lotus position.
But with the kneeling chair, your legs are fixed. Your upper body, is like a joystick, a tree swaying in the wind. You can't readily get off since your legs are weight tied. You have to do a dismount.