They need to be well trained and disciplined because they're a very strong breed. If you focus their energy in the right way, though, they're great pets, just as any other dog.
There's a common conception of big dogs as aggressive and needlessly hostile, but that just isn't the case unless the dog has been poorly treated or received next to no discipline throughout its life. I personally have a German Shepherd - he's a rescue dog, and despite his obvious intelligence and character, I could really see when I first got him that he needed a lot of training to be a good pet. The more you put into making your dog a good pet, the more you'll get out of it in the long run, regardless of the breed.