I've used the wind/mechanical version whistles for years...not sure if they work or not, but I've never hit a deer either. I have seen deer off to the side of the road take off and run the other way while being on the bike. Now, with my regular vehicles (which I don't have the whistles on) I've seen deer either stand still or jump across in front of me. So, it makes me believe that there is something to running those things. A rider was killed a few years ago, not far from where we used to live, after hitting a deer. Shortly after that is when the whistles went on my bike(s).
I won't ever ride without them...but, I don't let the confidence factor of having them enter into my riding style either. I tend to ride keeping in mind that I'll be the one to find the only deaf deer in the state crossing in front of me.
By the way, our insurance agent/carrier (State Farm) has given them out for free in the past...one might check with one's agent/companies to see if they do the same. They must be of the opinion that they work.