Here's a watchout for the Plantronics 655: if it goes in and out of pocket, including briefcase pocket the silicone ear piece can easily turn to the unlocked position and fall off.

It seems that the locking mechanism is much better in the beginning of use and becomes less secure as it gets worn in.

It's a PITA. I don't like walking around with the thing in my ear, but in and out of jacket pocket means once in every 3 - 5 times I come up with the unit minus the ear piece.