I think you'd be hard pressed to find any manufacturers out ther who would knowingly stick their necks on the line and state that this type of product is designed to be a 'stopper'. The law suits would follow very quickly.
As was stated, it is primarily designed as another option to 'deadly force', as a deterrent to escalating threat, and to provide you with time to make an escape or to to turn the hand in your favor. A direct blast to the eyes/nasal/mouth area of most folks or animals would definitely slow down or terminate the aggression, as the aggressor would be in some discomfort.
As JD stated, it has some value in the hands of those who might be a little timid to pull a weapon, or think of using a weapon in a threatening situation. They are easy to use, no need to worry about where the safety is, lining up sights, etc. So for most folks they are a no-brainer to use, which in a panic situation is something that could be life saving.
I would definitely say they are USEFUL, not only for those folks but for many of us. I don't carry as much as I used to, but I would carry something like this if I couldn't carry my regular CW.
As for stopping PCP/dusted monsters, it might not stop the attack, but it would make it very hard for the monster to find you, as he would have a heck of a time trying to see and breath, much less worry about where you went. And sorry, but if it took that many shots to stop someone, then there are definitely some training issues involved. When I went through training, 3 shots became second nature to you. It was automatic, and never failed. Those three, center of mass (heart), head, and pelvic area (groin/hip if you could). Whether or not this would have stopped your friend's PCP superman, I don't know. But I'm pretty sure it would have given him a bad day.
But YES on the gas.
Slo