You Can Test the iOS 9.0.1 Update Out
For the moment, you can take the iOS 9.0.1 update for a test drive.
What we mean by that is, you can install iOS 9.0.1 and then drop back down to iOS 8.4.1 or iOS 9 if you decide that you don't like iOS 9.0.1's performance.
This downgrade loophole will only be open for a few weeks so you'll need to take advantage sooner rather than later. You also won't be able to drop down to anything older than iOS 8.4.1.