We have one of these:
http://www.amazon.co.uk/PestClear-Profe ... e+repeller
We had mice that were feeding quite happily with the poison (from the council pest controller who left them). So we cleaned up all the poison, have all of the holes blocked (or so we thought) and at the same time we put this repeller on. No sign of mice for ages. Husband decided to switch this off 2 weeks ago and droppings reappeared out of the sudden. So I switched it back on again and no sign of mice now.
The previous comment on ultrasonic only effective in the immediate area etc is true. This repeller has both ultrasonic + electromagnetic effects, with the latter driving the mice away from the house.
So I think it does work, yes.