I have three dogs.
Apollo - 110 lb Male Doberman, a blue one. He's a big baby most of the time but has gotten very nasty before when someone approached my wife too quickly. Didn't attack, but I had the feeling that if the leash was off...
Ariel - 70 lb Female Doberman/Rott mix. She's the alarm. Nothing gets past her. She's very wary of strangers and will not even accept food from one. She ignores it until one of us picks it up and gives it to her. A very useful trait.
Alexis - 75 lb Femal Doberman/Pitbull mix. She's the boss. Very aggressive, and totally fearless. We have to keep a close eye on her. She will not hesitate to attack and is very very territorial. If you come into her turf, you'd better look humble and acknowledge the fact that she's in charge by looking away from her. She doesn't play the stare game. Lock eyes with her for more than 30 seconds...and she'll put you in your place.
How do they react to intruders? Only have 2 examples.
1) The male was still a pup, so he wasn't in on it. Neighborhood troublemaker decides to hop the fence and attempt to make off with some tools. Ariel immediately sounds the alarm (she was asleep in the kitchen) Alexis charges out the pet door and gets herself a mouthfull of inner thigh (danger close). Ariel follows and gets herself a mouthfull of calf. Ariel releases after a few seconds, but Alexis bites down hard and holds on, even after several blows to the head with a 3/4" Ratchet by the BG. She finally releases once I got close enough for her to see me in her peripheral. BG tries to haul ass but falls off the fence and bangs his knee pretty bad. Cops arrive seconds later (amazing response time...less than 3 minutes) and arrest him. Charged with criminal tresspass, pled guilty, paid fine, picked up some trash, back on the street. Surprisingly, his injuries from the dog attack were very minor. They didn't even call an ambulance. Only a few stitches in his thigh, none in his calf.
2) Wasn't home, second hand account from neighbor. Ignorant juvenile delinquent ignores 3 "Beware of Dog" signs on privacy fence and opens gate. All three charge instantly, thinking daddy is home to play. Upon seeing stranger, their demeanor changes and they give chase. BG runs into street, dogs stop at property line as taught and bark for hours. Neighbor calls me. I go home, put dogs in fence. No bites. No charges. No lawsuits.
My dogs are a very important part of my family. I wouldn't trust them with our lives, but I feel my property is more than adequately protected.