Used to smoke heavily, 25 a day except on Sundays where it was around 60 as I would pretty much do the chain smoker's perpetual flame and light one cigarette with the last.
That is a habit I wish that I had never started in the first place, everyone says that you start to feel better after a week of quitting smoking. Absolute tripe! It was nearly 6 months before I felt better. Things got so bad that I went to a specialist and offered him cash to perform a lung transplant on me so that I could continue smoking and wreck a new pair of lungs. Let me just say that is one way to make a medical professional see the red mist "That's not how it works! Leave my office immediately!".
On a more positive note I went for a lung function test about a month ago and the nurse said that I really got away with smoking. Apart from treatable asthma lung function is nearly perfect.
I'd strongly recommend staying away from smoking.