My bf got bitten on the lip by an adult male beardie. It drew blood, but barely, and he had a purpley red dragon-mouth shaped bruise on his lip for a couple of weeks.
It was the day he got this guy who'd been given back to the pet shop cos the previous owner didn't have time for him, and the pet shop tried to handle him regularly but couldn't do it enough, so he was very skittish. He told me he was trying to touch him and the beardie turned round and bit him. He later confessed he was actually trying to kiss him, so I stopped being quite so sympathetic at that point, and laughed at him instead.
I wasn't there, so I'm not sure if it was a scared 'leave me alone' bite, or a full-on agressive bite, but I doubt very much a beardie would ever do serious damage to a human.
They can hurt each other a LOT though, I imagine if they can bite of another dragon's leg during a fight, they could get close to being able to bite off a human finger, but I think their instinct with us is to lash out a bit then run away, rather than go in for the kill, so you'd have to be seriously traumatising your beardie and not let him have any space to get away before you'd be likely to get seriously hurt.