One of the greatest endorsements a company can get is an investment by Warren Buffett, one of the wealthiest men in the world and an investor who has realized annualized gains in excess of 20% over his lifetime. To put it into perspective, $1000 invested in 1965, at the start of his fund, would be worth over $3.5 million today; that's an appreciation of 350,000% (this is not a typo).
In 2000, Buffett's Berkshire Hathaway (BRK) took a 17% stake of all outstanding USG shares. The company will issue an additional 45 million shares very soon at $40 per--doubling its current float--and the Oracle of Omaha pledged to buy any shares not purchased by investors. This could increase Buffett's share in the company to over 50% if demand for the new offering is low.
Like every one of Buffett's investments, this stock is for long-term value buyers. When the U.S. housing market picks up again--and it will some day--USG will be minting cash as does in any housing boom.