I think feedback cards are hit and miss. I say this because it depends on what type of gig you are catering. If you are doing a wedding or other type of event where they are not just gathering to eat, it is not about feedback at that time for the guest or you. I find I get the best feedbcak by contacting the client after the event.
Here is more feedback: use bigger fonts/points when posting, most of us are getting older here