I smoked a pack a day for about 3 years and quit in August.
Originally I started mainly out of curiosity and also for the social aspect (new to work, all my coworkers smoked, socialising during smoke breaks, etc); I've also had a history with stress and anxiety so it did manage to do a good job of calming me down during stressful times.
Then I decided to quit during easter of last year but my anxiety struck back with the wrath of an angry god so I postponed that notion til summer when I was off college. Then one day kind of said "Oh right, quitting" and just upped and stopped, no patches, no cutting down, none of that. I tell you know it was HARD. AS. HELL. they say the cravings stop after a week, bullshit, 6 months in and I still used to dream of smoking and wake up with massive cravings, but it's a willpower thing. Given the chance to do over I'd probably still start smoking, and I probably will smoke in the future again one day. Just maybe cigars for special occaisions though.
Why you ask? Because for all the cost incurred and those few days I woke up feeling like someone krav-maga'd my lungs out, it was still a very enjoyable experience. And since when does ANYONE stop doing something they enjoy doing because it can be harmful? Sky-diving is pretty dangerous but ya don't see massive campaigns trying to get people to quit and showing pictures of someone smashing himself into a fine pulp to use as a scare.
Bottom line; life's too damn short to worry about what might or might not kill you. You can be the healthiest person on the planet but one day you go out your door and bam, knocked down by a bus.