I'm not a fan of these at all. They are a good option between a physical fight and deadly force for a cop, but unless you have the back up plan of a firearm you are screwed. Cops mostly use this as an alternative to wrestling and cuffing bad guys, not for defense.
For the most part the laws that would allow you to use it would also allow you to use a firearm so you don't get to use it for less than deadly force encounters. If you face deadly force it isn't very good at it. Short range, one or two shots at best, if they are wearing thick close or can grab something to use as a shield the electric charge will never get to it's target even if the barbed darts hit.
In short it's like pepper spray and stun guns it may talk a bad guy out of an encounter or at least help them to pick a different target, but it's not a defensive tool of any great use.
My advice is if you can talk them into a firearm, great, if not get them a fair sized can of pepper spary foam which has just as much range and lots more shots.