In general, it is my opinion that it is best to get your nutrients from eating a wide variety of nutrient packed foods. The US Dietary Guidelines recently published agrees!!(http://health.gov/dietaryguidelines/2010.asp) . You can't replace all the fiber, phytochemicals, etc. found in fresh fruits, vegetables and grains by taking a pill. However, supplements have their place. Some people will benefit from vitamin and mineral supplements under special circumstances which are not limited to but include: preparing for/during pregnancy, post-gastric by-pass/other malabsorptive conditions, vegetarian/vegan, and severe food allergies. Some other supplements such as herbs do have science to back up some of their claims and others do not. If you want reliable info on nutrition supplements the National Institutes of Health (NIH) has a great online resource where you can look up common supplements and herbs to find out evidenced based information (info backed by science). Here is the link:
While vitamins and minerals are essential (you can't make them and you have to have them), you can get too much of a good thing. Everything in excess can be toxic. You can even cause death with too much water, it would take a lot, but it is possible. So keep this in mind, more is not always better!
Here is a great article from WebMd if you want to know more about how to pick the safest supplements: