I'm one of the ones that has had bad experiences of these breeds so I'm afraid I'm very wary of them and do give them a wide berth. I live in SE London and unfortunately many owners of Staffies and Bull Terriers in this area (we're talking Old Kent Road, Peckham, New Cross etc) have them because they think they are macho dogs, do not socialise them, let them run around the streets and shops off lead (often without even a collar on) and generally aren't what I'd call responsible owners. As an example, in the last week I've seen a woman in the vet's with a Staffie pup who kept really smacking it because it wouldn't sit still, a man who was walking his Bull Terrier and it wouldn't walk to heel, so he kept yanking it back to his side and then slapping it round the head (and obviously thought this would make it want to walk next to him...), and a young couple in Pets at Home with a Staffie that frankly didn't look old enough to be having walks (it was on the floor) and was sitting down looking absolutely petrified and trembling while they literally dragged it around the shop and referred to the pup as "it" all the time.
My view is that I keep a very close eye out for these breeds and try to avoid Bella going near them if I can - she's been attacked 3 times by bull terriers (twice by dogs approaching her when she was on the lead - one of which pinned her down and stood snarling over her throat every time she tried to stand up and whose owner thought it was sweet how they were playing, once when she was 5 months old and I stupidly let her go bouncing over to a bull terrier which then grabbed her by the chest and did alot of damage - she fought back but 5 months on still has a huge lump of scar tissue on her chest) and that's set my mind for good, I'm afraid.
I know that any dog badly brought up can be dangerous - it's just unfortunate that alot of people in my neck of the woods buy staffies and bull terriers specifically because they are trendy and look macho, and they don't care about bringing their dogs up to be well socialised, so you end up with badly brought up staffies and bull terriers who are then aggressive to other dogs, and perpetuate the view that these are dangerous dogs. It's a shame because I think they can be lovely dogs in the right hands - I've come across a couple of really gorgeous bull terriers in my time who were the most placid, soppy dogs you could meet, but then again they had lovely responsible owners who adored them and brought them up that way.