When we bought Nogglestead, one of the outlets in the corner of the lower level had a Bell Howard Ultrasonic Pest Repeller plugged into it. It was out of the way, so we just left it there. For a very long time. Seven or eight years.
However, we have professional pest control services for bugs and quadrupeds for mice, lizards, and snakes. So about a year ago, when I was plugging or unplugging something from behind the chairs or perhaps doing one of our decennial vacuumings behind the reading chairs, I unplugged it and set it on the bar behind the coffee pot and electric tea kettle.
My own thinking: (1) this device comes from Bell and Howell, an old-line optics operation, and (2) you’d think an old-line optics operation would balk at having its name appear on questionable technology like this.
I doubted that it served its claimed purpose; the only mouse intrusion we’ve had was when one got in from the garage when we stored the cat food underneath the bar sink — right next to the repeller. The mouse didn’t have to cross any cat-patrolled ground for a snack, and I’ve not seen any other evidence of mice in the house since we’ve moved the cat food to a different cabinet. Nor did it keep out the various snakes, frogs, and lizards that the cats used to find (but they haven’t found in a while, which must mean the new cats are lazier than their predecessors, or the reptiles and amphibians are more cagey).
I own a similar device, and have similar questions about its efficacy. On the other hand, its placement makes it a good night-light.