It's been less than a week since iOS 9 was released, but it's already being used by more than 50 percent of iOS devices, according to an Apple press release. The 50%+ adoption rate in less than a week is, as Apple puts it, the fastest iOS adoption ever.
In addition, Apple reiterated its upcoming launch of the iPhone 6s, stating that customers may come to their local Apple Store on Friday, September 25th, as it will be fulfilling walk-in orders. Customers are thus encouraged to arrive early, as supply will no doubt be limited, especially if there's a specific color/size/carrier combination that you simply must have.