I own a beautiful sweet purebred English Mastiff named Jade. We specifically sought out a Mastiff based on our experience with them. They're called "Gentle Giants" by fans of the breed and it's because they are by and large the sweetest, most friendly breed. Yes, she's a drooling, bomb dropping slobber bucket, but she wouldn't hurt a fly and our family adores her.
It's funny, because I own a Chihuahua also, and she is 50 times more agressive than Jade ever thought of being. I have to be more careful of the Chihuahua getting herself into trouble with other dogs when we go to parks than the Mastiff any day. I suppose there are cases of agressive Mastiffs... but I'd have to believe it was a matter of gross abuse/negligence by a bad owner that would cause it.
Our drool machine with my daughter, she's not yet full grown here. They are great dogs to have inside... they just love to flop down and your feet and stay there.