After talking to multiple users over the weekend who were warned of an iCloud problem, we then met two new iPhone buyers who attempted to perform an iCloud backup to the iPhone 6s, and ran into problems.
There is a chance that the iPhone 6s will go into a boot loop when you choose an iCloud Restore at setup.
If this happens you will need to attempt to get to Settings and perform a factory reset on your new iPhone before it restarts, or you will need to take the phone back to where you purchased it for a replacement.
Just bought the iPhone 6s. Already have to return it to the store because it's unable to upgrade/keeps restarting the upgrade. Unreal.
— Kevin Olivieri (@KevinOlivieri) September 30, 2015
This issue is sporadic, but it is exactly why we suggest doing an iTunes backup in addition to the iCloud Backup.
We were able to perform several iCloud restores on the iPhone 6s and iPhone 6s Plus since the release date, without issue, but we’ve seen scattered reports of the issue.
We’ve learned that one way to get around this issue is to make sure that you upgrade both phones to iOS 9.0.2 before you attempt an iCloud backup.
If you plan to buy an iPhone 6s, you should upgrade your current iPhone to iOS 9.0.2 now. It isn’t clear if you need to complete a iCloud backup on iOS 9.0.2, but that would be a good idea.
When your iPhone 6s arrives you can set it up as a new iPhone and install the iOS 9.0.2 update. Currently Apple is not shipping the iPhone 6s out of the box with iOS 9.0.2 on board.
After you complete the set up as a new iPhone, go to Settings -> General -> Check for Update. This will install iOS 9.0.2 on your new iPhone in roughly 10 minutes.
After this completes, you can reset your new iPhone to factory settings and in the setup guide, choose to restore from iCloud backup. With both phones on iOS 9.0.2 you should be safe to use this option.
When purchasing an iPhone 6s for my wife over the weekend I ran into two users who were waiting after an employee was able to get in and do a factory reset before the iPhone 6s restarted again.
Although reports of the iPhone 6s and iPhone 6 restarting randomly on iOS 9.0.1 appeared in early October, this issue does not appear linked to the iCloud restore that puts some phones into a boot loop.
Install iOS 9.0.2 If You're Concerned About Security
If you're concerned with your device's security, you might want to install the iOS 9.0.2 update right now because it comes with an important security patch for a well-known passcode flaw that was discovered a few days ago.
Shortly after the iOS 9 release, iOS users discovered a passcode flaw that could allow someone to bypass the lock screen and look at contacts and photos. For some people, that could be a huge problem.
Forutnately, Apple's new iOS 9.0.2 update comes with a patch for the problem and we've been unable to replicate it with the issue on board. If you're concerned with this problem, iOS 9.0.2 might be worth a download.