First of all, I checked with the Met. Police who do NOT consider this to be an offensive weapon. That said, you'd probably find that individual coppers took a different view if you started using it on strangers.
This small, hand-held device weighs next to nothing but is made of a very tough plastic and is held with the 'pointy' end out between the index and middle fingers. The head looks like a large, blunt cross headed screwdriver and is made to amplify the pressure of a press, push or punch into a small area, hence the description of the Stinger - it's a 'force multiplier'.
I gave this thing a few tentative tests and quickly found that even a fairly gentle prod on soft tissue (muscle mass e.g. upper or lower arms, shoulder, pectorals or around the ribs) produces much more pain that you would expect. In the event of having to use it in earnest, I have no doubt that a sharp push or jab would be enough to make an assailant stop short and a hard punch (or more) with this would render the most determined attacker unable to think of anything but the excruciating pain just inflicted.
Obviously, as a self-defence device, it should be used as a means to prevent further attack and to enable escape and, I believe this little tool would be a big help in achieving that end. Add a Jet Scream whistle for maximum effect.
Hitting an someone hard around the body is unlikely to do permanent damage or prove life threatening and if one's safety is under threat, I'm sure the reaction would be to hit hard enough to incapacitate the attacker. This tool would, almost certainly, allow that to happen.
I bought this keyring and a whistle for a grand-daughter who walks to work in West London at all hours. She tells me that she now feels more confident in her ability to repel unwanted attention, if the occasion should arise (I hope it never does).