Overall, its an elegant update that makes subtle but important changes to using the mobile products.
But one thing users need to do it to reset the "Hey Siri" option. "Hey Siri" lets users command Siri when the device is plugged in. During the iOS 9 upgrade, the setting is set to "off" even if it had been previously set to "on".
To reset "Hey Siri", go to Settings, General, Siri. Toggle "Hey Siri" to on. You'll then need to train Siri to your voice through a few screen prompts.
Apple has a good review of what's new in iOS 9 HERE.
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