I even have a friend who has one strapped to the front and the back when she is kayaking and hasn't seen a deer/horse fly near her ever since.
I wear one on my cap when riding/hiking and I haven't been bitten since.
So, for about $10, I haven't been bitten once in about 4 months.
It could be coincidence .... but I'm currently a believer.
It also seems to scare the annoying flies away (the ones that don't bite, but get in your mouth/eyes)