I have seen articles and heard people ask the question - Does Ultrasonic Rodent Repellant Units work. I would have to say - Absolutely!
We used to have problems with rats getting in our walls every fall - my mom bought one of these units from QVC and that was about 4 years ago - we haven't had the problem since.
Also, my grandmother had mice in her house, it didn't seem to matter how many cats or traps she had there always seemed to mice. After our success with the Ultrasonic Rodent Repellant Unit- we got her one. That was about 3 years ago and she hasn't had once mice since!