Very well indeed. In a press release issued this morning, Apple (AAPL) announced that it had sold or delivered more than 2 million copies of OS X 10.5 in its first weekend on the market. OS X Tiger, by comparison, took nearly six weeks to reach the 2 million mark. That makes the launch of Leopard the most successful OS release in Apple’s history.

“These numbers show the Mac user base is growing,” writes PiperJaffray’s Gene Munster. “It also shows that it is an unusually active user base, with 9% of the approximately 23 million users upgrading in the first four days.” He notes that there were half as many Macs in circulation in April 2005 when Tiger was released, yet it took Tiger nearly ten times as long to reach 2 million sales.

Comparative sales figures for Microsoft’s (MSFT) Vista operating system were not immediately available, but the company is said to have licensed 20 million copies in its first month, a number Leopard is unlikely to surpass. But that’s comparing apples and oranges, given the relative size of their respective user bases. Last week Microsoft reported that it had sold 88 million copies of Vista in nine months, representing less than 9% of the worldwide installed base of roughly 1 billion Windows machines.