Customer disatisfaction is commonly measured by customer complaints and by warranty & service information. Customer satisfaction may be measured by customer surveys, focus groups, telephone interviews or to a lesser degree by 'bingo' cards sent with orders.
Since we are talking about auditing for ISO systems, the new revision ISO 900x: 2000, requires that we measure satisfaction therefore it is not an either or situation.
By far, the bingo card is the least expensive but has the disadvantage of a set format. In addition, you can expect a 5% to 15% return rate. At the other end of the spectrum is the cutomer focus group which yields extensive specific information but is extremely expense. Customer surveys are a good compromise yet are still expensive. Customer survey returns are usually more than bingo cards but still not extensive.
A good place to start would be understanding the dynamics of customer satisfaction. Noritaki Kano developed a customer satisfaction model called appropriately, The Kano Model. You might want to start by looking at it. Here is a reference to start from
http://www.servqual.com/kano.html. Also there is a Scholtes book reference at
http://deming.ces.clemson.edu/den/archive/98.05/msg00109.html, Hope this helps
