*cough, mumble* "Had to open my mouth.."
Well for lack of a better way to put it...
THEY HURT
if you get hit by them, but not to the degree that it's a real threat.
It's more like getting sucker punched. The soft exterior does a lot to cushion the impact.
At a recent 3D shoot I was slinging alongside my fellow bowmen, and hit a 3D target with a weighted tennis ball by way of experiment. The target had a small dent the size of the tennis ball, with a small rupture along one edge. The blunt trauma seemed nasty, but there was no penetrating like rocks do.
The point is however, you probably won't kill someone with this, and people don't know these balls are weighted, so they aren’t afraid of them.
'What they don't know can't...' you know the rest.
-Unsapien