iOS 10.3 uses lots of mobile data
If you've noticed that your mobile data allowance is quickly being used up after upgrading to iOS 10, then the Wi-Fi Assist feature may be the culprit.
Wi-Fi Assist was introduced with the iOS 9 update, and it enables your iPhone or iPad to tell when your Wi-Fi connection is poor. If it is, then the device will automatically switch to your mobile data. While this means you can happily browse and download without experiencing any slow down or loss of connection, it can also mean that you're using up your monthly mobile data allowance without knowing it, and it seems like the feature is turned on by default with iOS 10.
To turn it off, go to Settings > Mobile and scroll to the bottom of the menu. There will be a switch next to 'Wi-Fi Assist', so make sure this is turned to 'Off'.