My family has a shared 6GB data plan. Typically we use 2-3GB per month. In fact I had recently been thinking of downgrading the plan. On Saturday I was surprised to discover that for this period my phone had consumed 16GB of data. I am the only one in the family on iOS9. All this data usage has been since I installed iOS9. It has totally been blowing through my cellular data without me knowing it. Investigating I believe the cause is the WiFi Assist option which Apple has turned on by default. This little cherry is buried in the Cellular settings (at the very bottom). When switched on (as per default) if iOS9 thinks your Wifi connection is not very good it will use your cellular data. So you may be thinking you are on Wifi watching that latest cat vid on YouTube when intact you are consuming your data plan. I now have to call Verizon and dispute this data overage. I'm a little mad. You should check your data consumption. You may be surprised on what you have consumed in a week.