In a word, very. Dividend income has represented roughly one-third of the total return on the Standard & Poor's 500 index since 1926.*
According to S&P, the portion of total return attributable to dividends has ranged from a high of 53% during the 1940s--in other words, more than half that decade's return resulted from dividends--to a low of 14% during the 1990s, when the development and rapid expansion of the Internet meant that investors tended to focus on growth.*
And in individual years, the contribution of dividends can be even more dramatic. In 2011, the index's 2.11% average dividend component represented 100% of its total return, since the index's value actually fell by three-hundredths of a point.** And according to S&P, the dividend component of the total return on the S&P 500 has been far more stable than price changes, which can be affected by speculation and fickle market sentiment.