In 20 minutes your blood pressure will drop back down to normal.
In 8 hours the carbon monoxide (a toxic gas) levels in your blood stream will drop by half, and oxygen levels will return to normal.
In 48 hours your chance of having a heart attack will have decreased. All nicotine will have left your body. Your sense of taste and smell will return to a normal level.
In 72 hours your bronchial tubes will relax, and your energy levels will increase.
In 2 weeks your circulation will increase, and it will continue to improve for the next 10 weeks.
In 3 to 9 months coughing, wheezing, and breathing problems will dissipate as your lung capacity improves by 10%.
In 1 year your risk of having a heart attack will have dropped by half.
In 5 years your risk of having a stroke returns to that of a non-smoker.
In 10 years your risk of lung cancer will have returned to that of a non-smoker.
In 15 years your risk of heart attack will have returned to that of a non-smoker.
Source: Google Effects of quitting smoking.
I can confirm most of that myself (of course I can't prove myself that my heart attack risk has dropped) but my energy is better than with smoking, my coughing is gone and the best effect is smelling and tasting. I can't tell you how often I taste/smell things smokers don't taste or smell, it's really funny.
I have quit smoking quite easy, it never had been an important status for me, it was purely our of boredom and a bad habit of course. I mean if you use your logic you can't deny that smoking is bad. It's smoke! You are willing to breath in smoke that you would naturally avoid, your instincts even tell you that it's bad. (Smoke from fire, pollution etc.)
A little tip if you have a hard time quitting smoking: Try quitting when you are sick, most people don't smoke when they are sick. I know this sounds stupid but the best time to stop smoking is when you are really feeling bad. (surgery, bad cold) That's mostly because your body has more important things to handle than your little stupid addiction, so it won't bother you with "I need to smoke".