I don't know if they really act as a deterrent or not, and I don't care much to get involved in that debate. All I know is that when I carry I feel like at least I've got a fighting chance should something go down. I don't necessarily feel unsafe if don't carry, just as I don't feel safe if I do. I just feel more prepared.
I follow the rules and am a good person. I know chances are I'll never need my gun, and the one day I do will likely be the day I die, because I'll be in a Gun Free Zone.