You Can Downgrade So Don't Panic
Keep in mind, you should be able to downgrade your iPhone 6 or iPhone 6 Plus to iOS 8.4.1 in the week or two after the iOS 9 release date.
Apple typically signs off on older updates for a week or two after a new update arrives. This leaves a downgrade loophole open. This loophole will allow you to back down to iOS 8.4.1 if iOS 9 doesn't perform well on your device. And it may not.
This article may contain affiliate links. Click here for more details.