Omaha, Neb. ? Warren Buffett remains optimistic about the prospects for his company and the nation even though Berkshire Hathaway Inc. turned in its worst performance in 2008 and the widely followed investor says the economy will likely remain a mess beyond this year.

Buffett used his annual letter Saturday to reassure shareholders that the Omaha-based insurance and investment company has the financial strength needed to withstand the current turmoil and improve after the worst showing of Buffett’s 44 years as chairman and CEO.

Buffett wrote he’s certain “the economy will be in shambles throughout 2009 — and, for that matter, probably well beyond — but that conclusion does not tell us whether the stock market will rise or fall.”

In between the news of Berkshire’s sharply lower profit and a thorough explanation of its largely unrealized $7.5 billion investment and derivative losses, Buffett offered a hopeful view of the nation’s future.

He said America has faced bigger economic challenges in the past, including two World Wars and the Great Depression.

“Though the path has not been smooth, our economic system has worked extraordinarily well over time,” Buffett wrote. “It has unleashed human potential as no other system has, and it will continue to do so. America’s best days lie ahead.”

Buffett’s letter appeared to mollify the concerns of many who follow the company, but it’s not yet clear whether that will help Berkshire’s Class A stock extend its rebound from the new five-year low it set last Monday at $73,500.

“If anything, I feel better than I did before I read it,” Morningstar analyst Bill Bergman said.

But Buffett estimates Berkshire’s book value — assets minus liabilities — declined 9.6 percent to $70,530 per share in 2008 — the biggest drop since he took control of the company in 1965.

Berkshire’s Class A shares remain the most expensive U.S. stock, but they fell nearly 32 percent in 2008 and have declined 48 percent since setting a high of $151,650 in December 2007.