The health benefits of stopping smoking start almost immediately:

After...
20 minutes: Blood pressure and pulse return to normal
8 hours: Nicotine and carbon monoxide levels in the blood are halved, oxygen levels in the blood return to normal
24 hours: Carbon monoxide is eliminated from the body and the lungs start to clear out the build up of tar
48 hours: There is no nicotine left in the body. Taste and smell are greatly improved
72 hours: Breathing becomes easier, bronchial tubes begin to relax, energy levels increase
2 - 12 weeks: Circulation improves, making walking and running a lot easier 3 - 9 months: Coughs, wheezing and breathing problems improve as the lungs have room for up to 10% more oxygen
5 years: Risk of heart attack is halved
10 years: Risk of lung cancer is halved
10 years: Risk of heart attack is at the same level as non-smokers
Medium to long-term: Risk of developing lung cancer, other cancers, heart attack, stroke and chronic lung disease is reduced - the sooner you stop, the sooner your risk starts going down
Medium to long-term: Stopping smoking at any age increases your life expectancy, provided that you stop before the onset of serious disease. Even if you have developed a disease, you can benefit from stopping as your body will be under less strain and be able to fight it more easily
Medium to long-term: A smoker who has suffered from a heart attack can halve the risk of a second heart attack by stopping smoking
Medium to long-term: Ten years after stopping smoking an ex-smoker's risk of lung cancer is reduced by 30 - 50% compared with that of a continuing smoker
Medium to long-term: Smokers who stop before the age of 35 have a life expectancy not significantly different from that of a non-smoker
Medium to long-term: Smokers who stop between 65 and 74 years of age have a better life expectancy beyond 75 than those who continue to smoke

For life:

Reduce stress
Any physical activity is easier
Don't tire as quickly
Improve sense of taste and smell
Improved skin