A.D.
I have one and thankfully have never had to use it. I figure for such a low price, I might as well buy it. Check out the ResQME tool which is a window punch that works underwater, a seatbelt cutter and a keychain all in one. It's like $17 which is a small price to pay if you ever find yourself in that position.
http://safetycentral.com/rememiliha.html