Actually, I have trained with being on the receiving end of these. And no they are NOT useful for personal self defense .
Ignore everything you've seen in the movies or TV, it's greatly exaggerated fiction .
In the real world, you not only need to get up close and personal, you have to manually have them restrained, such that you can hold the device in good contact with them for 5 seconds plus . Just a touch and a zap movie style , they'll just jerk away from you , and either beat feet , or be ticked off at you enough to really start punching you .
It's actual narrow niche of usefulness is more in a custody situation.
They refuse to enter / exit their cell ? Dogplie them with C.O. and get the 5 sec contact while they're pinned , to get they temporary loss of muscle control , then handcuff them , and drag them into / out of cell .
Or on the street several Ofc are present with non comply ING subject . Resists cuffing . Insufficient reaction to OC .But threading the gap where not justified to shoot , and not prefered option for ASP Shampoo .
Couple of Ofc tackle , hold against fixed surface , whatever is sufficient to hold him still , 3rd Ofc apply device for 5 seconds, then cuff , and drag to cruiser .
For private individuals defending against typical attacks , " Step 1 - Place attacker in Full Nelson and immobilize him " is not realistic or relevant.
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What IS useful is a genuine X 26 Taser ( the one that shoots the barbs with attached wires , from 20 feet away ( or a bit more). It works, from a relatively safe stand off distance .
The catch is the cost , $500 ‐ish .