I have heard that they can be a real handful for anyone who doesn't have a flock for them to guard, but I have never met any personally. They sure look cool, though.
I knew someone who had one. The dog was very aggressive around anyone he didn't consider family. He even bit her husband once because her husband wasn't home enough to be come the part of the dog's "flock".
They definitely are a dog for an experienced dog owner as are all the flock guardian type dogs. They have to be very well socialized as puppies, obedience trained, and get a lot of exercise. They do best in the country where they have room to run so if you have one in the city you will have to put the time into giving them daily exercise. Even given everything they need by an experienced owner they probably will never be a cuddly lovey type dog, that is just the nature of a guardian type dog. They are beautiful dogs and can be good companions provided they are the personality type dog that you want.