Apple announced this morning that iOS 9 has been installed on "more than 50 percent" of active iOS devices since it was released last Wednesday. The company says that this is the fastest adoption rate ever for a new operating system.
This is certainly a change from iOS 8's adoption rate a year ago. After around three weeks of availability, that operating system's share had stalled out at around 47 percent. Two months after that, it had grown to 63 percent, a number that iOS 9 should handily be able to beat by then. Even iOS 8's adoption rates were better than what you get from most other operating systems, but they were considerably lower than those for iOS 6 and iOS 7.
The iOS 9 update requires less free space to install, one possible explanation for why users are installing it more quickly than they installed iOS 8. It also includes significant new features for the iPad, has gotten good reviews that have praised its stability, and doesn't significantly slow down existing iDevices.