When targeted at and distributed to the right people, with the right message and at the right time, direct marketing is the most cost-effective way of communicating to existing and potential customers.
Carried out correctly, it is also the most measurable of mediums. End-users have never had so many sources of supply, and suppliers have never had access to so many markets.
Planning (data analysis) and execution has to be spot-on to maximise relevance though. Far too much direct marketing gets one or both of these wrong.