As part of my police training I had to make contact with a "suspect" which resulted in being sprayed in the face with pepper spray (18%). I then had to knee strike/take down an instructor who came at me (he was wearing protective gear). After that I walked "briskly" over (with other instructors pulling me for some reason) to a man sized target at approx 14 feet away, and I had to shoot until the instructors told me to stop (7 rounds in a 4 inch circle, not bad ). To finish it off I had to radio dispatch about what just happened.
This is what I learned:
First off, Pepper spray's effects are completely subjective. Prior to being sprayed numerous of my friends told me that you would be completely unable to keep your eyes open after being sprayed. They also said it was absolutely horrible.
In my situation pepper spray was completely ineffective. After seeing a video tape of myself, I noticed that I initially did a shrug of my head (Maybe in a sub conscience attempt to shake it off) but other then that you would have no idea what happened. I went into the situation with absolute certainty that I was going to keep my eyes open the entire time. I did close them initially (I heard you couldn't open them after closing them for the first time) but after that I had them open. The pain wasn't anywhere near what people made it out to be. I did have a rather nasty taste in my mouth (Identical to the taste found in many really hot hot sauces). After 15 minutes the pain started to get a bit worse in my left eye because some of the spray ran into the corner of my eye. The bright sunny day outside really didn't help either.
I found out how effective it can be against some people. I ended up going back and helping other class mates through it (Directing them to water and a fan). Many of them were crying and almost 100% incapacitated. All of them fought through the same scenario I did however, which proves your still capable of a lot even after a direct spray to the face. Overall I would say that pepper spray is effective in 80 to 85 percent of cases that its used properly, in my experience.
What to expect if your sprayed:
Initially you will feel a rush of heat. Followed by a sharp pain (Similar to a sun burn). Standing in front of a fan eliminates the pain almost 100% from what I hear, I can vouch that the wind from running a bit helps a lot. Water does almost nothing. Expect lots of snot, in quantities you would think are impossible. This is good because the snot will cool you face. The effects will last around 30 to 50 minutes. The sun will hurt bad, much like a sunburn. I never touched my eyes, which might have been why I fared well. I say many people rubbing their eyes, which could certainly have caused issues.
You can fight through it, and yes you can keep your eyes open (Again I did, however most people in my class couldn't, they had to hold their eyes open to take shots). Pepper spray should never be used if a deadly force situation is present, unless its your only option.
EDIT: To ad a bit of information:
Most of the people I have talked to say that it would be hard for them to fight not so much due to the pain, but because they couldn't see anything (Aka couldn't keep their eyes open). In my case since I could see 100% (Minus the fact I didn't have my contacts in which obviously affected m vision) that wouldn't be an issue thus I (and anyone else that could keep their eyes open) would still be a significant threat.