I've been in law enforcement for over 21 yrs now, with 14yrs and 11 months spent working in a large jail prior to going out on the street (and yes I did actually enjoy it, kind of a sadist i guess), anyways I couldn't begin to guess how many times I was "spontaneously" attacked and I responded with pepper spray (10% solution), I will tell you it was more than once, and in both the spontaneous and the "prepared" responses it only failed once to assist me in subduing the "BG" and that was traced back to a bad batch from the factory (as we had several non-performance instances from that same case of spray).
Yes I do agree that you have to train in both the use (what reactions to expect from the "sprayee") and how to deal with the overspray (for when you and your partners get to experience it at the same time), but it is a great tool, when used properly.
I also feel that for someone who isn't prepared to, or wants to, carry a firearm it again is a lot better than nothing. Not everyone (civilian, or for those who want to be politically correct, non-law enforcement) can or is willing to train for impact weapons or empty handed defense. So as long as they are willing to realize that there is no "perfect weapon" and that it is a tool with limitations, I again think it can be a great option for them.
On a side note when I first started back in 1987 we used CS spray and that I found did have a lot of failures when using it on combative intoxicated (alcohol/narcotics) or mentally impared inmates. In the 90's we (the sheriff's dept) switched to the 10% pepper spray and the city P.D. switched to a 5%, with the city experiencing alot of failures (so much that when they brought in a combative inmate who they had sprayed they were still fighting, but when we hit them with ours they immdiately would cease, and it only took one time). Every time after that when they would return to our facility as soon as they entered into the Lock Up they stopped fighting. I couldn't tell you how many times I heard " I don't want any of that **it"