I eat a lot of olives and did a big study on whether it was better to buy pitted or unpitted olives. My conclusion after exhaustive research eating thousands of olives was you should always buy unpitted olives and pit them yourself.
I had been using this cherry pitter to pit my olives, but of course it didn't work very well. Then my wife bought this Cherry & Olive Pitter from Oxo and it is the bomb; it works beautifully! No splatter, easy to clean, and consistently smooth pitting.
I eat $13 worth of unpitted olives a week, so this thing is a steal at $13.
UPDATE (Jan. 2018): I'm on my third one of these as the previous two broke, so expect to replace it every quarter if you pit a lot of olives like I do.