I have to say, I've tripped kids wearing them. I know it seems mean, but a)they're banned b)they're annoying. And even if I don't trip them, most of them end up falling within ten feet anyway. There's too many people to be zooming around like that. It's just plain dumb. I saw a kid fall of the stairs at Mexico because he wasn't paying attention to where he was going, just skating along, then WHAM! down the stairs, head first into the concrete.
And really, all I think of when I see heelies is "wow...that kid's too lazy to PICK UP THEIR FEET AND WALK!". Yes, it's mean, but it's also kinda true.