I burn diesel so I don't do the Costco wait for gas. At my club, lines are often backed up onto the street making it nearly impossible to access the club's parking lot (or get out). To make it worse, the club is landlocked. On average, they are $.25 per gallon less than the street price.
With the typical car having a 16-gallon tank, and assuming most are about 3/4 of a tank low, that makes their gas purchase about 12 gallons or a savings of $3 over street prices. Waiting in line, on average about 20 minutes, it isn't worth it to me.
I've noticed that Costco has noticed that their gas islands need a better design. In newer built clubs I've recently been to, (Queens Creek AZ, and Melbourne FL) the clubs are located in a light industrial parkway setting with massive fuel islands consisting of four island runways with 6 pumps and room for additional pumps. The pumps also carry diesel, unlike the existing older Costcos.