[Gophernut]

We got one last summer for a lab that would just take off and not respond to verbal commands. We have had to zap her once. She took off, I gave her a beep and then a low shock. She stopped and immediately came back to us. She has since had to only hear the tone to stop the unwanted behavior. We now only occasionally have to put it on her, and usually only when there will be other dogs around. We live in the country, so it's nice to be able to let her out to run when it's too cold for us to stand outside. And even then, the collar is so loose that the tone is the only setting that would work anyways.
The zap setting is so low, that my daughter's fiancee zapped his arm, and said it felt like a 9 volt battery to the tounge. More of a tickle then a zap, but enough to get the dog's attention.