They work okay. Best use is their relative velocity. What I mean is that if you use the ball and it says you throw an 80 mph FB, and a 65 mph curve tha spread is probably relatively accurate. The actual number may be off.
The other problem with them is that they don't last forever. WHile they can take an occasional bounce, they aren't indestructible. Also - if they get wet and stay wet for long, they're toast! They don't last in the Oregon humidity!!