Quality of Vitamins

I have heard from other experts that the quality of Centrum vitamins is not high quality. 

What is the importance of high quality vitamins?  What if the Vitamin is some high percentage of filler?  What if the vitamins are not pure?  As long as thh vitamins are not mxed with carciniogins or poisons, what does it matter that a vitamin is poor or high purity?

What are the other standards of quality for vitamins?