Holders of Sirius XM (SIRI -0.21%) shares have been sorely disappointed over the years. Since the company's initial public offering in September 1994, the shares have gained just 8% compared to the S&P 500's 969%. That should make many investors cautious, particularly those who have lost money or badly lagged the market. A look backward can prove instructive in understanding a company's present situation and the market's assessment. But it's important to take a forward-looking approach. Have the markets sig ...