I have the ball chair that I use at home and one for my desk at work and also the cushion that you are talking about for when I travel to retreats etc.
I like to put a cloth pad over mine it makes it a little more comfortable to sit on, for me the rubber makes me sweat and my legs get sore. It does ease the pressure on my lower back and makes me sit straight. After a while I like to switch to a regular chair for a while. Changing around seems to help then I am not in one position for too long.