I'm a big believer that there are no bad dogs, only bad owners.
That said, dogs are bred for a wide variety of characteristics, and popular breeds tend to be 'brittle'... as they get overbred. Cocker spaniels are a great example - a wonderful sporting dog, wildly overbred, and what was once a dog with a generally excellent temperament is now very often snappish, high-strung, and prone to a variety of illnesses.
IMHO, pit bull terriers are not inherently a bad breed. However, they are bred for strength, aggressiveness, and size. They are a popular breed, and often overbred or poorly bred. They are popular with owners who don't train them well, or train them to be aggressive. I think anyone who approaches any large, powerful, unknown dog is crazy, but I also think there's a large difference between doing so with a pit bull and, say, a Bernese.
I'm very cautious with Pits and other large, powerful dogs in person, and at the dog park. More than one off leash that's anything but obviously very well-trained and well-behaved is probably a sign that it's time for me and my (very large, but ludicrously passive) dog to be somewhere else.