Your views on Mastiffs?

I have heard such different view's on mastiff's, you search on web and are told, although big english mastiffs/american mastiffs fantastic with kids and great family pets then I hear on the other side don't do it they should be banned they are too aggressive? Every mastiff owner I have spoken to have no trouble with their dogs! Why is there such a big divide in opinions on these breeds? I have recently got an american mastiff puppy and have 3 children. Having had big dogs all my life I believe it is the way they are brought up but I do believe certain breeds eg rottweilers do have a natural aggressive streak (just from experience of being bitten by one and my dog attacked by one). Your serious opinions greatly appreciated!