Individual investors can boost their portfolio returns by following some of the most well-regarded investors. Warren Buffett, arguably the greatest to ever do it, gets a lot of attention thanks to his legendary track record. 

If we take a closer look at Berkshire Hathaway's portfolio, we'll notice that one of its top holdings is American Express (NYSE: AXP). It's the third-largest position, worth $25.6 billion (as of July 31). Buffett first purchased Amex shares about 30 years ago, and it appears he remains bullish on the business. 

But before investors rush to buy the company, here are three important things to know about the financial stock.

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Source Fool.com