I've never been hit by one, but I have seen them go off...I've seen someone with 2nd and 3rd degree burns across their whole face, forearms, and upper chest from an airbag. The airbag went off as she was pulling out of her driveway for no reason. She got lots of money from the car company.
I knew a lady who got a massive black eye and a bruised rib from being in a 30 MPH wreck, thanks all to the airbag...Yeah, it was pretty intense. She didn't come in to school for 2 weeks and she still looked pretty bad when she came back.