So tomorrow the public will be able to get iOS 9 and it’s important to remember that the first several hours are always busy with massive server overload so update times tend to be longer than normal.
If you have a developer account you can install the GM version now so you don’t have to wait, if you don’t have a developer account you might want to hold off a bit so you don’t spend hours waiting.
If you are Jailbroken and want to stay jailbroken DO NOT update to iOS 9 there is no Jailbreak for iOS 8.4.1 or iOS 9 and there is no way of getting back to a jail breakable firmware. If you are jailbroken and haven’t done so already install Cydia Impactor to help you if your device messes up, also install NoPlsRecovery from angelXwind repo to prevent accidental updating.
If you do update to iOS 9 and hate it then there is only a limited time in which you can downgrade (guide here) to iOS 8.4.1 if you prefer it but again there is no Jailbreak for this version.