More than half of Apple devices have already downloaded the new iOS 9 software, making it the fastest iOS transition to date, according to Apple.
There are many improvements, most of which are subtle, such as an the extra hour of battery life the update gives you, and the improved connectivity between apps.
But if you don't have an unlimited data plan, one new setting could make your phone bill soar through the roof.
If you don't have unlimited data, you could be affected
The update introduces a new feature called "Wi-Fi Assist" - which will use your data to boost Wi-Fi whenever a router signal is weak.
It is designed to ensure that you always have a satisfactory web connection, and automatically switches your phone from Wi-Fi to cellular data.
However if you don't have extra data to spare, it will need to be manually turned off, or you will risk going over your agreed allowance.
The sneaky switchover to the cellular network could mean that your careful balance of Wi-Fi versus cellular use is doomed.
To turn off the setting simply go into your Settings then to Cellular, then scroll to the bottom, and toggle off "Wi-Fi Assist."