Always without fail. Lost a very close family friend back in december 2006, side impact shot her across the car which caused severe internal injuries that took her life at just 17.
It makes me angry seeing people not wearing them. If someone in my car refuses to put one on then I refuse to give them a lift simple as. Not a problem now that I've taken the rear seats out though lol!
Next time your in car and it's safe to do so, brake HARD at 30mph. Not fast at all and there's no impact (if there is, you've done something wrong lol
), but you have vertually no control over the movement of your body.