I grew up in a place where lots of people including some I knew personal literally smoked and/or sold crack.
I tend to groan a bit inside where I hear someone make a remark about "smoking crack" because it's often used to describe behavior that is quite unlike that of a genuine crack smoker.
Guy I know started tweeting increasingly erratic things over the course of a 48 hour bender, followed by to-the-whole-company emails about working remotely from Fiji so the CIA couldn't follow him. Didn't recognize me when I tried to meet up with him to get him some help. Later he apparently tried to light his apartment on fire.
I'm not sure I've met any, but if I extrapolate from the blue-collar folks I was exposed to I'd think it might be hard to tell.
Anecdotally, of course...
You've probably heard of high-functioning alcoholics, it's entirely possible to a high-functioning crackhead as well - at least for a while.
I expect the point of exposure for most people comes not through continued use, but the point when something interrupts their supply. That's where they end up crushingly depressed, violently angry and indebted in ways that will surely screw-up the rest of their previously well-functioning life.
I tend to groan a bit inside where I hear someone make a remark about "smoking crack" because it's often used to describe behavior that is quite unlike that of a genuine crack smoker.