Never used this myself in anger - but an old buddy did once - in UK - heck, he had few other options in that hell hole of zero self defence! It supposedly had ''significant'' deterrant effect - that's all I recall.
Longish sock - fill with a good handful of loose change. Whirl about head and swing at BG - whack - ouch!! Sounds to me potentially effective - plus I guess you'd say easy to conceal.
But - any thoughts? Effective? I rather think it could be.