I'm an instructor... I carry one every day (a two-shot X2) version. These are expensive units, that cost over 1k, plus annual battery replacements and daily spark testing to keep the inerds "seasoned" or whatever the hell that means (likely just a way for Axon-Taser to sell you more $70 batteries). That all said, these are rugged and dependable, but STILL only get maybe a 50% effective rate in the field. As stated above, factors like thicker clothing, suspects that break the wires, or plain old "ya missed" are key factors. But when they work, they work well. Civilian units are specifically designed to be deployed, create disfunction in the attackers muscles for like 10-30 seconds, then you lay that shit down and run away (not even joking).
The handheld "stun" guns do nothing but pain compliance. That is they just hurt. They don't do anything to create disfunction in the muscles (other than the exact spot they strike). Tasers work by creating a neuromuscular incapacitation between the probes (which is why they spread apart when you shoot them out). They literally create static that just fires all the muscles and you can't control yourself, fall down, etc.. The stun guns just jolt you at the point of impact, and most raged attackers can and do fight through that. The movies sow people getting stunned and passing out for an hour, lol... gotta love the movies...
As stated, if you're looking for a SD tool, OC is nice, and less "threatening" than a taser. However, awareness and mindset are the biggest factors of avoiding be targeted for attack. I teach this at work, and do a demonstration of two potential victims. I have the entire lecture hall put on imaginary burglar masks, then pretend to look for victims. I walk in front of the class twice. Once with a meh, nervous energy, and once with a confident and aware vibe. It's palpable which person the class chooses to attack. Criminals follow the path of least resistance. Be very obvious to everyone watching that you are not the person they should pick to rob/attack. This doesn't mean walk around with your chest puffed out and flexing, it means be confident, alert, and unafraid.
OK, lesson over, that will be $19.99 payable to me via paypal