In my experience, the deer whistles do work...provided you keep them free of bug carcasses and whatnot.
If they're plugged up, they won't whistle.
The CHP use them and swear by em.

The night before I bought mine, I was rolling at about 40mph on a two-lane road out of Tiburon. A full-grown buck jumped down from the embankment about 150ft in front of me, and only by the grace of God did that deer jump back up in time. I literally had no way out in that situation. Needless to say, I bought a set the next morning. With the whistles fitted, the deer were leaving long before I was near. Never had another close call.

For five bucks, I'd go ahead & buy em.
Unless your species of deer are tone-deaf. h: