My wife and I foster rescue dogs. Living in Baltimore, this means primarily pitbulls. The pitbull isn't actually a breed, it's a general term for a variety of breeds with similar features. Before this experience I had very little experience with these breeds and I like to think I'm pretty objective.
I have now "got to know" dozens of these dogs, from a variety of backgrounds. My wife, who is more involved than I am, has spent a decent amount of time with probably 100+ animals. The bottomline is that very, very few of these animals are at all aggressive. In fact, in situations where aggression would likely be displayed, they tend to avoid it more than other breeds I have worked with - they look at you like "Are you going to make me be aggressive here? I don't want to".
And therein lies the problem. They're very very loyal, very intelligent and very easily trained. That makes them ideal if you want a fighting dog, because they will learn how to fight easily and will fight to the death if their master wants them to. But they don't want to. You can see their opposition to it in real life, and you can definitely see it in dogfighting videos if you are ever unlucky enough to view them.
The above qualities are why some segments of society unfortunately train their dogs to be aggressive. And because they then favor those breeds, the breed earns the reputation it has. But these same qualities mean that a rescued pit can usually be re-trained, with a higher success rate than other breeds. That, together with dangerous dogs being identified at the shelter, means that very very few that are adopted out ever go on to cause trouble - again, less than other breeds even.
Basically, pit bulls are absolutely not aggressive dogs. They are just favored by people who train them that way, and that is only a small percentage. Should you be scared of them on the street? Be cautious, sure, but you should do that around any breed when you don't know the individual animal. Don't blame the breed though, blame the owner.