I have one...
but I dont have it on the fence because I am worried it would draw attention. "hmm, look at that. I wonder what kind of dogs are in there?" or "oh you can huh? sounds like a challenge." On the fence I have a warning sign on each gate. One says "beware of dog" and the other says "beware, guard dog on duty". I figure those are more warnings, where that one is more of a challenge, or draw for idiots or immature teenagers/kids to get close and peak over the fence.
also have a magnetic bumper sticker that says "my german shpeherd is smarter than your honor role student".