the only reports I've read where they were found to be somewhat effective was when the vehicle carrying them was driving directly towards a deer (or herd of deer) in the middle of a field. Apparently, if the vehicle was driven at above about 40 mph, the deer would scatter as the car/truck approached.
... but the question was never answered - was it the whistles - or the fact that there was a speeding truck approaching them that made them scatter?
I'd second Knifemaker- save your doe (no pun intended) for better horns and lights.
I'd also second Phil - that most animals' fight or flight reflex (or startle reflex) fires more strongly by audible alerts than visual ones. That's why they play the scarey music suddenly in movies to scare the be-Jesus out of you.
Bottom line - if it makes you feel safer - I'd say, fill your boots. I wouldn't recommend that you place a great deal of faith in them doing anything though. If the daft buggers (deer) can't / won't respond to the noise of an in-line 4 at nominal speeds, why would they bother with a little whistling tone played atop the same in-line 4 whine?
PS - I read a note recently that said, if you clip a deer with your bike/vehicle in the States, it's perfectly legal to eat the bugger. Only fitting end to an other wise bad day if you ask me. A little olive oil, some fresh cracked black pepper .... hm-mmmmmm! If you do have to finish him off with the old AlpineStar motocross boots, make sure no-one sees you.