Recent evidence suggests that it is far better to eat a healthy diet rich in fruits and vegetables than taking dietary supplements, states a great review in the New York Times
http://www.nytimes.com/2009/02/17/health/17well.html?_r=1&ref=health Vitamin Pills: A False hope
Results from the Womens Health Initiative that looked at multivitamin use over 8 years in 161,000 women showed no benefit, this was also shown to be the case in men, in the Physicians health study. In fact large does of vitamins can be harmful in the case of Beta carotene, increasing the incidence of lung cancer in smokers.
The latest vitamin in the spotlight is Vitamin D, a study is underway at Harvard to see if it can reduce the risk of cancer and chronic disease in 20,000 men and women. So watch this space.
The bottom line is to eat whole foods not pills as its the synergestic effect of all the nutrients present in the diet that seems to have the benefit.
There is and never will be a substitute for a balanced diet.