What if your customers tell you they are liking what you do but give you no insight into whether they actually like what you're trying to accomplish.
For example, I assume there is some non-zero part of the population that derives real value from 'junk mail' (the paper kind) and phone books that are still delivered to houses across the nation by transforming them into great fire starters and fuel for indoor fires.
On surveying that audience, you'd likely hear glowing praise for the practice and encouragement for more. Survey everyone else and they'll likely toss your form of neatly arranged 'on a scale of one to five' questions into the nearest recycle bin.
A more sensible approach (which I'm sure is common practice) would be to measure conversions and really see how effective direct mail campaigns are. I question how easy that would be to do with a phonebook however, so perhaps the annual yellow pages still has a few years of opportunity left yet.