Very effective. Even if a big share of your ad impressions falsely target someone who already bought (see sibling comment) the remaining impressions lead to an

Very effective. Even if a big share of your ad impressions falsely target someone who already bought (see sibling comment) the remaining impressions lead to an increase in conversions at a comparatively low cost per conversion. As you said, this will vary from business to business, but I have seen very successful retargeting campaigns in b2c e-commerce as well as b2b lead generation.