I'm highly sceptical of such things but my mum swears by hers - I think she paid $40 for two and she seems to think they keep the insects out (didn't have a mouse problem)

My dad and step mum live on a farm and have many scattered around and seem to think they do reduce the mice somewhat (in comparison to the numbers beforehand) 

To me they're a bit like the plug in EMF neutralizers that are big in australia