If you have any question about whether you should carry it or not, or if you are not familiar with it at all, then don't. More people I know have tried to carry that stuff, and set it off accidentally. Outside of police work, I can't say that I know anyone that's ever used it in self defense. I know a lot of people that have tried to carry it as a self defense tool, only to have it go off in a purse, a car, etc. It makes a nasty mess. I've been covered in the stuff many times. It's not a pleasant experience.
Another problem with it is outside, sometimes it's not extremely effective. If you're not real close in, the wind can carry a bunch of your stream away, and it does take a little bit before the effects of the spray materialize. Depending on the distance, this could pose a problem. It doesn't take a lot of the stuff, but I've been hit with it, and it seems to take 15-20 seconds before you really start feeling the full brunt of the spray. That's long enough for someone to take it from you while you fiddle for something else (too long). I sprayed a guy with some OC, and he asked me if I had some he could put on his pizza. Another problem with the spray is indoors, everything will get covered. If it goes off indoors, clear the room for a while.
Given the issues with the spray, this makes people consider the foam. I sprayed a guy with foam I had to carry once. He was bald. He turned his head to the side, and the foam caked up on the side of his head. He scraped it off and asked me what else I had. Not cool.
The best uses I've ever found for any kind of spray were when I was physically fighting with someone, as in I'm on top of them, or close in hitting them, and I can get to the spray. I give them a quick squirt right across the eyeballs, nose, and mouth. And by this, the nozzle of the spray is usually no more than 2-3" from their face. I've never sprayed and not gotten any on me, so this is a fact of life with it. Expect to be fighting, hacking, coughing, spitting, and eyes water. Oh, and when you take a shower, lean way over and wash your head and areas that were hit first. The spray washes down your body. It's always funny to hear someone that's been sprayed for the first time in the shower. That stuff burns everywhere that it touches.
Personally, if I'm about to be confronted by someone, and they are 20 feet, even 40 feet away, I'd rather go ahead and verbally confront them (even if it's a "Can I help you?"), and adjust my body language to let them know I'm not going to be an easy victim. If the situation goes any worse, I have an ace in the hole that I haven't played yet.
Pepper spray is a waste of time when you're on your own. It does make for a great laugh and story though, when it goes off in your car on the way to work.
My opinion.