Yeah, I get a little sceptical about those things as well.
As far as plugging in one or more of these and expecting the pests to go screaming from the premises in a mass exodus - then that's unrealistic. I do accept, though, that the marketing for these things paints that picture.
However there is one explanation of them that I feel is plausible... that these things make the environment less comfortable for the pests and that, over time, the pests will find being somewhere else is more pleasant. How true that is, I don't know, but even if it were, I doubt it would be sufficient to surpass the offering of food and shelter.