Very useful for CAD & CAM software users. Unique in its kind.
I first saw it in the field I work in. At first, I was hesitant, but after a period of use, it became addictive (in a good way). The zoom, rotation, etc. it offers in CAD environments is something else.
The only negative is the price and weight (where you leave it on the desk, it will stay there), but it is justified due to its uniqueness.
Note-Tip: Consider it as an investment if you use such software and want it for professional use at home.