My definition of a bad product would simply one where the purchaser would not have bought it given the information they learned after acquiring it. A simple way to measure how good a product is would be to see whether the product is recommended by the purchaser, or whether they buy similar products from the same brand/source again.

Ah so you don't mean good and bad in terms of morality or the harm it will bring to the user but solely upon consumption preference. By this definition crystal meth can be considered a good product.