I was really impressed with this, easily put together, comfortable, maneouvreable, lightweight and adjustable. It makes you sit up straight and prevents you from crossing your legs, so you tend to sit with knees 45 degrees apart and feet firmly on the ground. On recommendation from my physio I also use it as a guitar stool, for which it is perfect, preventing me from sitting lopsided.
However, like kneeling stools it is probably optimistic to think of sitting on it all day, so the ideal solution might be to have this and a kneeling stool and swap regularly. I strongly recommend the rocker kneeling stool as it creates a kind of back massage effect. (NB. a kneeling stool is no good for guitar playing!)
I tend to slightly agree with the guy that says this doesn't suit the male physique that much, but more because it can get a bit hot around the groin. If men can ride horses they can sit on this!