This once iconic aerospace company has fallen from grace. Is now the time to buy its beaten-down stock?With shares down by almost 50% over the last five years, Boeing (BA -0.78%) has dramatically lagged the S&P 500, which is up by nearly 80% over that same time frame. Despite a deep competitive moat and a virtual duopoly in the market for large jet airliners, management has been unable to turn these advantages into sustainable shareholder value. Boeing's quality-control and legal challenges are dragging dow ...