Yes sir, they are loyal to their owners but aren't good with strangers, like you say. They are really smart, so training them isn't easy because they don't always want to do what you tell them.
I've only come across one and that was at work. He was one of our clients he needed a muzzle because he didn't like us, but then again, we've muzzled nice dogs because who wouldn't be angry at strangers holding them down and pulling blood? XD
His name was Yogi xDD