Presumably it works as advertised (has anyone actually tested one of these?), but there is one aspect of this hammer that could severely limit its effectiveness. As the image illustrates and how it was packaged, the pointed end of the hammer is sheathed with a flip-down silicone cover, probably to prevent inadvertent injury to self or scratching of a car interior.
However, after flipping off the sheath it flips right back up when you take a swing with it, interfering with the point and severely blunting the hammer blow. I tried it on cardboard and it produced a dent but not a proper puncture as with the cover removed. One would have to have the presence of mind in an emergency to flip off the sheath only to have it bounce off the window, not good at all. It's also very difficult to flip off the sheath and nearly impossible to remove it if you only had the use of one hand.
I cannot recommend this product unless you turn the sheath around to flip upward (much easier to open with one hand) or remove it altogether before you actually need it. Then practice using it. And be careful how you handle it if you do remove the sheath.
I'd give it four stars if it shipped with the cover flip up-able and not down-able.