As a Wildlife conservation officer, I carried and used pepper spray on a few people and on a variety of dogs and a few wild animals---in my experience, it WORKS and works well.
In order to carry it , we had to be trained to use it and know its affects so we were sprayed with it---it worked on every one at the training school.
It is said that something like 1% of the population of the USA is immune to its effects but I have not seen or met anyone in that 1% yet.
Although it is possible to get some of the affects on you yourself as the user, for me anyway it was always minor exposure---nothing compared to what the bad guy or offending animal received.
The last time I used it was on some Rottweilers that came after me while serving a warrant---they both got a direct blast and the last I saw of them they were running around blindly and rolling on the ground--they forgot all about me.
Just make sure you keep a fresh can and not something that has been around for 3 years in the trunk of the car.
Buy an extra can and try it out so you can learn the spray pattern and its actual effective distance.