From what I've seen and heard OC's effectiveness varies greatly depending on the ammount of the exposure and the individual in question it's used upon. Having participated in and observed OC's use in a gas house (an enviroment where everyone participating is exposed to a roughly ( I would say) equal ammount) I can tell you that reaction varies from person to person. Some people will be totally incapacitated (bent over coughing, gagging, crying, with snot hanging out of their nose four feet), while others will only experience an uncomfortable burning and constriction and once out of the enviroment will be not much worse for the wear, so I have to conclude that individual makeup counts for something as far as OC goes. Although it does seem to have at least some effect on everyone.
Exposure ammount will vary in real life also(Direct hit/partialhit/outside/inajailcell etc.) Anyone with any sense isn't going to stand still and let you hit them directly with that stuff, and also I think controlled breathing and not panicking also help in dealing with the effects of the gas.