My expereience with staffies are the small stocky ones and they are incredibly friendly and great with children. They are easily stolen though cause they go to anyone.
My experience with pure bred american pitbulls are good as well (this dog was papered, came from america and and it's papers were about an arm long). He wouldn't any human, my blue cattle dogs bit more people then he did Although I wouldn't trust him with another animal.

I think the problem comes from cross breeding and how they are brought up. I would hate to see what a pitt bull cross blue cattle dog would be like for instance too much power with too much brain/cunning.