Anecdotal 'evidence' is the best kind
I've had a couple of cheapie Bunnings sourced mouse upsetters plugged in for the last few years - and we haven't seen a mouse since! Of course it could be the power of positive thinking as well, or karma, or maybe just that I also sealed up some holes they were getting in and that worked (potentially interesting side note - if you have holes in brickwork or cupboards where pipes/etc come through, I've found the cheap wire dish scrubber pads make excellent hole fillers and it seems rodents don't eat stainless steel. You can buy bulk packs of them very cheaply and they shape to the hole well - I've used them where dishwasher hoses go through cupboard sides and in tight spaces where I didn't want to use filler and make a mess).
We'd previously had a lot of mice, but they do come and go with the seasons and local conditions, but it must be the deterrent surely? oh, we also have a cat now, but surely its the sonic deterrent?
Yeah I'm not convinced either. Reckon they'll be in the bin soon. I can't hear the things, neither can the cat, or the partners dog as far as we can tell ... hence a great deal of scepticism on my part, even though I wanted to try them
Re the car mounted devices - there's lots of talk, and one person I knew some time back swore by them until he discovered it was wired incorrectly and not working at all. Dumb luck had been his friend the whole time! even when (apparently) roos had been scampering away from the sonic warning beacon - he could see them reacting, it must be working - so funny.
The bird cannons some vineyards used in years gone by (do they still?) seemed to work for a while, but as I understand it the birds became accustomed to them ... unlike the people they annoyed ...
I do have devices that emit loud noises and neutralise the problem of rodents and (cross thread warning) could be used as personal alarms - but they exhibit the side effect of high velocity lead residue and have licencing/public use issues
Lots of good points made in other posts - not the least of which the apparent complete lack of scientific testing/proof they work coupled with a lack of uptake in industry/agriculture, it seems ...