I don't know if they really work but our police department swears by them. Statistically this anecdotal evidence suggest suggests that there is no difference. I do not know if studies have been performed. Maybe you can call the company and they can cite a study or maybe your local sheriff can help. As it turns out, this HAS been studied, and those little whistles do NOT affect deer at all.
Deer have a relatively low frequency range of hearing similar to cows.
Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.