Within the first 48 hours, usage rates for OS X 10.8 were already rivaling those of Mac OS X 10.4 Tiger. While over 45% of users still operate on Mac OS X 10.6 Snow Leopard, anybody using 10.6.6 or higher is eligible for the upgrade to 10.8. Those who skipped the upgrade to Lion, which had mixed reviews, may be more inclined to make the jump to Mountain Lion.
With 3.2% of all Mac users upgrading to OS X Mountain Lion in the first 48 hours, Apple's latest operating system stands to do much better than its predecessor OS X Lion, which experienced only a 14% market share after 3 months on the market.
The low price point and easy update process is likely why Mountain Lion is seeing such quick adoption.
You can find instructions on how to install the new operating system here.