These products work largely on the same principle as the Mosquito in the UK: a high pitched noise that can usually be heard by those under 25, but not those of us that are above it.
I couldn't really hear anything myself, as you can probably imagine from my previous copping to impending decrepitude. Happily, some of my friends have kids, so I could test things on them.
The adults largely heard nothing, though a couple of women seemed to have exceptional hearing and heard the noise. Those that heard it reported that it was annoying as all hell, sort of like the hum of a fluorescent bulb at its most annoying. Though one person said that it was was worse than a dentist drill and complained of a headache afterwards.
Other than possibly using this to give myself some space on the bus after school lets out, when roughhousing kids make the ride annoying as hell, I don't really see much use for this app.