It appears that Sam's Club really has some competition. In fact, according to the article below, the Costco business model seems to be much more endearing to the Costco employee too. Happy employees are less likely to file lawsuits (ala Walmart), and more likely to provide better service.
While I have never shopped at Costco, I do believe that it does have a window of opportunity to steal business from the evil empire (aka Wal-Mart).
I think Walmart has become more interested in expansion and profit margins and less interested interested in customer satisfaction.
How Costco Became the Anti-Wal-Mart - New York Times