I don't think I've ever been bitten. If anything maybe one of the different babies I had over the years took a snap at me. I'd never give big ones the chance. That's how you lose a finger.
I once had about 85 juvenile allies on my hands. I let some of them bite me; even a little one can give a pretty good pinch, maybe draw a drop or two of blood. I haven't been bitten by a common. Like VF said, you take whatever precautions you need to to be sure the big guys don't hit you. I've seen what they can do to a fish or piece of fruit; I don't want that to happen to my hand.
I've never been bitten and I hope to keep it that way. Snappy has snapped at me a lot, but I'm always pretty careful how I handle her (seems weird to say "her").
Once on the show Jack*** Manny put a pretty good size snapper up to Steve-O. It latched on pretty good, Steve-O screamed, and Manny got it off him. It drew a little blood and you could tell it caused some major bruising, but it didn't take a hunk out of him.
red-eared slider nailed me once. not fun, and i had to shake my hand to get him off. i was like 8-9 at the time. i was showing the terrapin i caught bare handed to some kids but it had a leaf on its nose and i went to take it off...