I've never had any issues with Costco on five BMWs over 8 years.
Costco near me has good flow (clean filters) and are very busy (fresh gas).
BTW, they recently reformulated their gas:
http://www.costco.com/gasoline-clean-power.html
"As our valued member, you expect Costco to always offer the finest quality products at the best possible price. Gasoline is no exception. Costco has closely studied fuel additives and engine deposits, and conducted extensive fuel tests at nationally-recognized laboratories. As a result, we decided to increase the detergent additives in our fuel to provide a better value to our members. Cleaner engines run better, run longer, and they pollute less.
Costco gas with Kirkland Signature Clean Power detergent additives contains five times the EPA detergent requirement. Clean Power doesn't just keep your engine clean - it also helps remove any existing deposits. Costco Gasoline - Something good just got better!"