currently have one (no pics though, sorry). best dog i've ever had. robust, athletic, relatively maintenence free. he's lived with me both in the city (in an apartment), and out in the country no where he is basically free to roam when i'm home from work. had no problems with either situation.
he IS a one-animal-family kind of dog though. he doesn't get along real well with other animals. though he won't seak out a fight, he'll stand his ground pretty much no matter what. they are also a one-man-dog kinda dog too. he'll listen to me and do whatever i want (though begrudgingly at times), but my girlfriend might as well be talking to a wall. if she tries to tell him what to do, or even scold him, the most she'll get for a response is a look.
if you are looking for a "security" dog, you really can't do much better than an american bull. that's pretty much what they were made for. i'll probably never have another type of dog again. in fact, even my ARFCOM scree name is about my animal.