I have a few comments.
1) Pepper spray is a fairly effective weapon in diminishing an attacker's capacity to attack. It will not stop a determined attacker, but it will reduce his/her ability to see and breath, and will be a significant distraction.
2) No handheld weapon is very useful if it is not held in the hand. It is not news that the weapon must be in the hand before the attack or it will not likely be of any use.
3) I used to work on a counter-drug task force where we put our lives in danger on a daily basis. In some kinds of situations we went in unarmed. We insisted that we needed some kind of protection for these types of operations and demanded vests. Instead, we got firearms. Unfortunately, mine wasn't big enough to cover all of my vital organs.
My point here is that people often mistakenly consider a weapon as an instrument of self-defense. Weapons are offensive instruments. (Yes, of course they can often be also used for defense, but you get the point.)
Rule #1 of self-defense is to be where the attack is not. Effectively, pepper spray gives you an extra margin of error provided you have it in hand at the time of the error. What I mean is that if you were unsuccessful at keeping yourself out of trouble, pepper spray can give you the extra edge you need to get yourself back out of trouble. It of course won't slay an attacker.
Throughout my training (military, law enforcement, Kung Fu) I've been sprayed, zapped, gassed, etc by pretty much every non-lethal weapon used. Pepper spray takes a pretty direct hit to do more than just aggravate the eyes, nose, and throat.
Peace,
- B. A. Carey