I have an ASP Key Defender on my off-duty keychain. It provides an intermediate level of force between using personal weapons and the lethal force of my handgun or knife. Not all encounters justify the use of lethal force.
If you're going to carry pepper spray (OC), the Key Defender is small enough that it doesn't become a burden. It's simple interface (pop off the safety collar, aim the end at attacker, and press the plunger) is easy enough in stressful situations. I prefer the stream of my full-size canister when in uniform, but the spray from the Key Defender isn't too bad in terms of range.
If you're not familiar with the effects of OC, know that OC most likely won't stop the attack. Know that even if the attacker is sensitive to OC, it takes several seconds for the OC to even take effect. A lot can happen in several seconds.
Many people have many different reactions to OC. I've had subjects immediately give up the fight, and I've had subjects that didn't even blink when sprayed.
If you're lucky, the attacker will shut his eyes and grab his burning face allowing you to attack or disengage. If you're less lucky, you'll have to fight off the attacker until the effects of the OC take effect to some extent or another. If you're even less lucky, the attacker will be able to fight off the effects of the OC and continue his attack. And in some cases, the attacker will not be affected by the OC whatsoever. Time to go to Plan B.
OC, when properly used, is better than nothing. But please don't rely on OC as a sole means of defense. Treat it as part of an overall system of personal protection, one that includes situational awareness, threat avoidance, defensive tactics, and use of available tools.
Hope that helps.