We have friends here in the U.S. that have two American Bulldogs and I've been around them since they were puppies. They bear out Woof's observations... however, these dogs are rural dogs and gets lots ... LOTS of exercise. They are exuberant because of their strength and intelligence.
As puppies they'll drive you up the wall since their exuberance is usally displayed in vigorous jumping... not a lot running though. Additionally, while they're short coated and easel groomed, not given to excess shedding, they do drool and slobber, like most bulldog breeds. They are also gassy... all the jokes about blaming the dogs for the flatulence certainly fit these guys.
Protective to a fault, especially of young children. I got into a faux-wrestling match with their 8 year old and when he started yelling, I was suddenly confronted with a less than friendly looking brindle and white face very close to my own.
So, unless you can exercise them vigorously for at least half an hot and preferably more, every day and find it difficult to oversee them with a firm hand when young, I'd probably pass... but not because they're dangerous!