Most are not dangerous to humans, but for a while they were specifically bred to be dangerous for dog fighting, so the more aggressive ones were chosen to pass on their genetic material over others. As such, pit bulls tend to be far more aggressive to small dogs and other small animals than other dog breeds. My girlfriend has one and I would not dream of taking him to a dog park.
A handful are also unpredictable and can snap and bite humans with their stronger than normal jaws. This has led to pit bulls having disproportionate rates of harming other humans compared to most dog breeds. For that reason, they are banned in many jurisdictions.
Neutering the dogs early tend to decrease their tendency to be aggressive though.
Are they dangerous? Yes. Every large dog breed is dangerous. Are they more dangerous than the typical dog breed? Yes. Is it likely an in-fixed, badly trained pit bull might attack a human? Yes. Is it likely a neutered, well socialized pit pull will attack a human? No, but you should always assume they can.