It helps that Microsoft is selling the upgrade for $39!
4 Million in 3 Days
MG Siegler on Windows 8's weekend numbers:
That's 4 million in 3 days. Solid, right? Sure. Though it has to be noted that Apple sold 3 million copies of OS X Mountain Lion in 4 days. Microsoft likes to poke fun at the small OS X install base, and now it works both ways. Apple got nearly as many people to buy the update for OS X in the same amount of time despite a sliver of the overall footprint.
Microsoft's has an installed base of 1.25 billion users. According to Ballmer's own numbers, 4 million upgraded to Windows 8 in the first four days. That means that one-third of one percent of Microsoft's user base upgraded.
Now let's take a look at Apple. In its first four days on sale, Mountain Lion sold 3 million copies. Apple has an installed base of 66 million, meaning that 5 percent of Apple's user base updated in four days.
This is a really hard comparison to make, though. It speaks to the differences in Apple's and Microsoft's customers more than it does the differences between Windows 8 and Mac OS X.