Are centrum vitamins good for you? are they natural or mostly synthetic? Do they get absorbed by your body?

Lots of people are into health and fitness and getting into shape and eating greens and lots of people I know are supplementing themselves with multivitamins like centrum of GNC or Puritan Pride per se , but are they really good for you, What is it that we should look for ? are they really absorbed by our systems or are they going down the toilet together with your urine without any benefit at all? Which ones are really worth ingesting ,anyone with any medicine based answers? Thank you. CG.