Just happened to see them use a spring-loaded glass puncher on a repeat of MythBusters yesterday. They were surprised at how effective it was (keys/kicking car window did not work).
Also great for those long road trips when the kids just won’t shut up.
Seriously though, every driver should have one of these. You never know when some distracted driver swerves her car and sends you into a conveniently located body of water. This may even save your life.