I had purchased this from Amazon before Christmas. It uses a 9v battery and I kept forgetting to purchase one. After reading this, I got the battery. I WAS AMAZED! I have tried the pennies in a can, my Maddie just barked louded to make sure she was heard over the noise. I tried ignoring her... didn't work. Tried the water in a squirt bottle, this worked to some extent, but I will admit I am lazy. We live in town and when it is a nice day, especially weekends, a lot of people walk on our street or ride bikes. This is TOTALLY unacceptable to my Maddie. Evidently the street in front of our house is her street. Anyway, getting up to go into the other two rooms with windows that she can bark in front of everytime someone walked by was driving me nuts!!! again.... lazy..
Anyway, put the battery in and waited. Kids walking by, my Maddie goes nutso barking. One quick press on the "controller" and she stops barking and looks at me like wth was that? I say "quiet" and then "good girl" "good quiet". She never started barking again. Next time someone walked by, same thing. I was amazed. It has been raining this week so I haven't been able to use this again, but with the weekend coming up, we shall see. So far, I am cautiously optimistic.
My only question would be how it would work to scare away another dog coming at you while out for a walk with your baby, cause yes, the other dog will hear the noise, but so will your furbaby...
Edited to say: it did not seem like it "hurt" her at all, just seemed like it surprised her, she looked around to me and cocked her head.