From GoSquared’s real-time Mac OS X version distribution page:
The graph shows the percentage of pageviews tracked across all sites on GoSquared which are on each version of OS X at a minute-by-minute granularity.
According to GoSquared’s data, after just three days, Mavericks is already installed on 1 in 10 Macs. Interestingly, it looks like most of the early upgrades are coming from Mountain Lion, which was the current version of the OS prior to Mavericks. The share for the other four versions (Lion, Snow Leopard, Leopard, and Tiger) has remained essentially constant since Mavericks was released. Will users of older versions of Mac OS X upgrade to Mavericks now that it is free, or is there something other than price (e.g. lack of awareness, laziness, fear of bugs) that is holding them back?