Initially is was amazing, then immediately it wasn't. The bearings barely rotate now. Guess I have to buy WD 40 to fix the issue....
I've used another brand of rotating cup for about 5 years. It works well to rotate the ball in one plane. Hammer's version rotates freely in all directions since it uses captive ball bearings that can move 360 degrees. Also, this ball cup has very "sticky" feet that keeps the cup firmly in place.
Doesn't matter if you're a Hammer brand loyalist, this does the job better than others. Initially, I've seen this cup advertised at up to $29.00; by pre-ordering I saved $1.00 (!) off the $19.99 price.