iOS 9 was only released last Wednesday and hit the 50 percent adoption mark on September 19, just three days after it was released.
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To compare, iOS 8 reportedly took about eight days to reach 50 percent of users. That release likely saw slower adoption as it contained fewer major features and caused difficulty for iPhone owners with 8 and 16 GB devices.
iOS 9 includes new features such as Proactive Siri, Apple Maps transit directions and improved battery life.
Apple also said in its press release that the response to iPhone 6s has been “incredibly positive” and confirmed that US stores will have limited stock of the handset on September 25.