I have come to learn that your most critical customers are your most valuable. Not only do they give you an honest perspective on your current service levels but more importantly an opportunity to make right what you are getting wrong in your business. You should see every customer criticism as an opportunity to learn and embrace to make your business better.
Regular feedback in the form of satisfaction surveys and one-on-one discussions will yield valuable information that you can use to better all of your clients experience of your business.
Your surveys and feedback need to include a very important question that determines your net promoter score i.e. would your customer recommend you to his friends and business associates. This is increasingly becoming a standard agenda item for board meetings and is one of your company vitals that should be regularly checked. Put together a simple online survey in a couple of minutes and start getting your feedback.