I have two Dobes, I think they would bail anyone up and bark at them if they are outside. When they are inside however, they are pretty relaxed and less likely to do anything unless the person was acting suspiciously. My male has a very solid character and would not be scared off by confrontation and I hope he would protect me, but the 1 yo bitch is more timid but normally backs up the male when they bark at strangers.
I saw a doco about a dog trainer simulating a break and enter in a number of dog households and the persons confidence around dogs actually put most of the dogs off!