Costco's (NASDAQ:COST) unconventional business model remains highly successful despite living in the age of Internet retailers. Paid memberships remain central to Costco's business model not only for profitability, but also for driving repeat visits. In this video, Motley Fool contributor Steve Heller goes through what makes Costco a great business, and whether the threat of Amazon (NASDAQ:AMZN) Prime is more than just a threat to Costco's business.