While smoking used to be romanticized and seen as a sign of maturity, I tend to see it as a sign of weakness and lack of willpower.
However, it depends on their age. If they are from older generations then I don't think much of them; it was part of their culture and they're just sort of "victims" of lack of information, though I shake my head at people who deny the cons or make little effort to at least try to stop.
If you are younger (no real specific age, maybe mid-twenties to 30?) and you smoke, I can't help but lose a decent chunk of respect for them and think they are incredibly stupid. There is absolutely no benefit from smoking, and the health benefits have been around long enough that anyone who is that young and smoking is basically stupid enough to be saying "Look at me, I'm causing permanent damage to my body, weeeeeeee!".
So to get that out of the way, if you are a very infrequent or social smoker I'm not really bothered by it, but otherwise it repulses me. :/