This tool is worth having. It easily removes stones (pits) from cherries and olives, and with little wastage. When removing the stones from cherries watch out for splatters. When pitting olives, put the olive pointy end facing up in the tool so that the stone is pushed out of the bottom of the olive (the end where it was attached to the tree).
As much as I love my olive pitter (or cherry stoner – the tool goes by both names), it does not function well on tight fleshed green olives. You are best to buy these ready-pitted or stuffed.