The wifes been using them for well over a year now,i know it sounds silly but apparently they do work, they create a bit of a racket in the drier, but that's in the porch so no real problem.
I've never used dryer balls but I have used those funny looking pink and green velcro ball things that go in the washing machine. They are supposed to remove hair/pet fur off your clothes. Absolutely useless waste of money.
Anyway, those dryer ball things look like one of those kids toys that bounce and bang around!