If you want to be a bad product manager, listen to customers only through surrogates. Why should you personally spend time listening to customers when you’ve got marketing research staff, sales reps, and customer service staff who do that all the time? You’ve got much more important things to do then to get out in the field and visit customers yourself. Make sure to read any feedback that’s coming in, skim the executive summaries of research reports, and you’ll be fine. And if your current resources aren’t telling you enough about your customers, there’s always consultants for that, right?
If you want to be a good product manager, listen to customers in person. There is no substitute for meeting customers face-to-face. Listening to customers where they use your product is most ideal, but even meeting at a neutral location like a coffee house is worthwhile. (Heck, even phone calls can be useful.) When you only get information on your customers through “filtered” sources, you miss out on a lot of the nuances that are important to learning about customer needs. Meeting customers in person gives you a chance to see how they use your product, see the environment in which it is used, probe on interesting areas and ask followup questions. This knowledge, combined with data gathered through other sources (sales feedback, market research, customer service records, etc.), gives you a very comprehensive view of your customers and is a powerful arsenal from which to draw for future product enhancements and new product ideas.