Yup, a very good friend of mine had one. He described the event as being kicked directly in the chest by a strong horse, completely by surprise (he'd grown up around horses and had a good idea of what a horse could do).
He was on a bit of a crusade to have the mfgr implement a pre-shock warning feature, just a buzz/beep/chirp/whatever a couple of seconds before triggering so he could prepare himself, as he'd found the shocks much more tolerable when he had an inkling they were coming.
AFAIK they haven't yet implemented it.
