It seems a LOT of people are still having trouble, because they aren't following harpreet0singh's instructions PRECISELY.
Lots of people are using iTunes to try to fix the problem (which is correct), but they're clearly in RECOVERY mode, rather than DFU mode, as harpreet0singh specified. THAT WON'T WORK, as it doesn't get you to where you need to be, and won't get around the "turn off find-my-phone" error.
Get the device into DFU mode (there are YouTube videos on how to do this if you're not sure how), downgrade to 8.4.1 (pressing restore in iTunes while simultaneously holding down the 'shift' key will show you software options OTHER than 9.0), and away it goes.
Once all that's done, iTunes will ask if you want to back up from your last backup, or start as a new phone. I chose to back up from a previous connection (November 2014), and let it run.
Surprisingly, it not only brought back everything that was backed up last year, but everything I've done since (including apps, contacts, photos taken only days before, etc. etc.). The ONLY thing I lost was the content of text messages I've sent since my last backup.
Am I going to try to bring my 4S to 9.0? Nope! It's working fine on 8.4.1 for the moment, so I'll wait for at least 9.0.1 or 9.1 before I take the chance again. And you'd better believe I'll do a full backup, first, before I say 'yes' to a software update!!