Surveys play a part in our customer feedback but so do reports from our salesmen, truck driver feedback, telephone contacts, daily correspondence, and other "daily interaction" but if it isn't documented and used in some way you are throwing away a good satisfaction indicator. To rely solely on surveys will give you an indication but I think you will lose the "real" feeling. Surveys are normally sent to a pre-determined "X" department and someone who may or may not have any idea might or might not fill it out and return it. The key with surveys is to get the right person to fill it out and usually the right person just doesn't have time for surveys. Develop a system that feeds contact reports from your salesmen/reps/drivers etc. back through your system. You can develop a value system to the contacts even if it is somewhat subjective. There is a lot of good information in "daily interaction" and unless it gets filtered to the right person, it won't help in determining trends in satisfaction or dis-satisfaction.
Dave