I smoked cigarettes in my youth but now find the habit disgusting. I quit -- cold turkey -- in graduate school and I have not touched a cigarette since.
I also find highly hypocritical to see people in the medical field who smoke. The nurses who are studying at the nursing school near my office take a regular smoke break right outside the building. Whenever I got outside for a break I often have to move from where I'm standing or sitting to avoid inhaling a wall of second hand smoke.