Have hit several with my cars, but only close calls on the bike so far.
I do not believe the deer whistles are effective at all. Maybe in the sense that it will give you comfort. In the studies I have read, they all come to the same conclusion that there was no real difference with or without. One thing that sticks to my mind is, that when I was in the academy for LE, they told us that after approximately 60 mph we were "out driving" the siren. In other words, studies had shown that over that speed, by the time other drivers recognized and reacted to the siren you were right next to them. So, even though deer may have better reaction time, it can't be that much. And looking at most, they are driven by your speed, meaning that the sound has to be redirected forward. Loss of time and volume.
Mark