@siina
It is great for me, better than Logitech Marble and those thumb ball ones. The trick to this and other equipment is to have the desk/keyboard pullout against your thighs so arms are around 90 degree angle or slightly lower. Of course if using it with your arm upwards the big size of it just emphasizes the discomfort.
The best part about the CST thing is that it doesn't require cleaning. Rolling the ball fast self-cleans it well enough. It is easy to open if need to actually clean it (I usually do every three years or so. Marble needed daily cleaning).
I'd suggest checking if you can order it with free return if not your thing.