Apple has rolled out many new mobile operating systems—nine, in fact. Some have gone smoothly, others less so, but each version brings new features or ways of doing things. With Apple's hyper focus on usability, that's typically a big win.
You could stick with iOS 8, which has plenty of great features. But after several months of testing iOS 9 with the public, Apple rolled it out to existing users in September with a few hiccups. It comes pre-installed on the new iPhone 6s and 6s Plus and the monstrous iPad Pro.
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