A Costco near me just opened a gas station and I've been using strictly Costco fuels on my cars for the last 2 months. They are consistently $0.20 cheaper than average fuel prices in the NYC area. I don't notice any change in my family's cars, except for one. My father has a 2006 Explorer with the triton V8. We know with 90k miles on the odo, the dreaded sparkplug change is becoming a necessity. I've had the CEL blink after going WOT previously so I know a tuneup is coming due. First time I filled it up with Costco 87, it started sputtering and bucking at anything over 3k RPM. I threw in some Regane drove it around cruising on the highways. After about a quarter tank down, I filled up with Sunoco 91 and all symptoms went away.
I habitually use regane about once a year on all my cars and as about 3 months of fillups at Costco will end up paying for a bottle of it for each car, I'll stick with Costco gas. But I don't think the detergent package is up to par with the big brands. After the Explorer's plug change, I'll try Costco 87 again on that vehicle.