We had one at my parents' house. It was installed for a strong-willed shar-pei and he would figure out how to get around it. He knew what the beeps meant and didn't care.

Gus, on the other hand, was also trained on it. He's a wuss. It hasn't worked in years (and he hasn't worn the collar in years, either) but still won't leave the yard. He's eight, and I don't think it's worked since he was a young pup.

Gus was also shocked ONCE by an electric fence in my grandparents' garden three years ago. He still won't go near the garden!