99% of the time they are bad. They tend to contain isolated, synthesized versions of vitamins created from coal tar or petroleum, or they contain toxic compounds, or they contain countless additives and texturizers and binders, or they are completely useless, or all of the above. Or, they lie on the label and don't put nearly as much of the compound as the label states. Consumer Reports has done testing on supplements and found that only a few brands actually contain the amount they say they do.
That being said, some high-quality supplements do exist.