iPhone 5 iOS 9 Release: 10 Things Every User Should Know

You Can Downgrade to iOS 8

If you run into iOS 9 problems, bugs or performance issues, you should be able to drop down to the iOS 8 update. 

Apple will almost certainly continue signing on its iOS 8.4.1 update. And if it does, you'll be able to downgrade from iOS 9 to iOS 8.4.1. Nothing older than that though. 

The downgrade loophole will be open for a limited amount (one to two weeks) of time so you'll want to look into the process sooner rather than later.