I've only had small scrapes or bruises, never any concusions, broken bones or anything serious. I'm pretty nuts about wearing protection when riding, and it's why I got my massive helmet.
The only serious fall/injury I've had was when I screwing around in a parking lot at my school, and a section of it had a large bump that sent my flying off the board and onto my side. Hip seemed fine though hurt like hell
Just got right back up and hopped on my onewheel and continued riding aound after a few minutes of cursing