My kids love their heelys. They wear them in the low-carpet basement of our house, and on our driveway/sidewalk. A helmet is required anytime there are wheels in the shoes, and the shoes are not worn with wheels in public places. That's just good logic, in my opinion. They are no more dangerous than the other recreational gear they have (skooters, bikes, inline skates, skateboards, etc).
The problem arises when poor parenting allows the kids to wear them, without gear, or worse, without gear in public places. Anyone who would let their kid wear heelys in a store is an idiot. Flame away for my honesty, but I'll never change my mind. Simply put, if you wouldn't let your kid ride their bike there, then they shouldn't be heeling.
Beyond that, they're fun toys that get kids moving. With supervision and intelligent parenting, they're perfectly fine.