Warren Buffett's investing conglomerate, Berkshire Hathaway (NYSE: BRK-A)(NYSE: BRK-B), had a fabulous first quarter, crushing analyst expectations with $90.9 billion in revenue (analysts expected only $86.4 billion) and reporting $8,825 per share in profit (for the "A" shares), 21% better than the Street's expected $7,309 per share. And how did investors respond?

They bid the stock up half a percentage point.

Half.

But not to worry, says investment bank UBS. Despite investors' underwhelming response to the news, Berkshire Hathaway stock is still a buy. In fact, "A" shares of the company that closed Monday around $606,000 will rise to the incredibly precise figure of $734,820 within a year.

Is Berkshire Hathaway stock a buy?

It may be only a coincidence that UBS raised its valuation on Berkshire Hathaway stock by exactly the amount on Saturday's earnings beat -- or it may be no coincidence. What's curious is that UBS recommends Berkshire Hathaway stock at all, considering the profits Berkshire earned were significantly less than what it earned a year ago.

As detailed in the company's report, profits per diluted share at Berkshire declined 64% year over year in Q1, despite Berkshire buying back 1.2% of its shares outstanding -- an action you'd ordinarily expect to concentrate earnings among fewer shares.

Most of the decline, however, was out of Berkshire's control, being in the form of a $33.1 billion swing in the valuation of stocks owned in Berkshire's portfolio... that the company bought a while ago and still hasn't sold. These unrealized gains (a year ago) and losses (this year) accounted for literally all of the decline in Berkshire's earnings.

Meanwhile, the company's operating profits actually increased by 39%, to $11.2 billion.

In other words, Berkshire Hathaway's core business continues to perform fabulously. Admittedly, the stock's not cheap at more than 1.5 times book value, and 28 times free cash flow. UBS's prediction of a price 21% higher would make Berkshire even more expensive -- perhaps even too expensive. But at a lower price, this long-term performer would indeed be a buy.