Following is the text on an email I've just sent to all the iPad & iPhone users at the organisation I work in:
Within a couple of days of iOS 9 being released to the world, the media began running stories about how iOS 9 would consume all your data and you should turn off the WiFi Assist feature. A couple of days after that, a class action suit was filed against Apple - demanding $5 million in reparation because of increased cellular data use…. Yeah, ‘MURICA!
A few people kindly pointed the issue out to me in case I hadn’t seen it - I advised them I’d prefer to wait to see what the actual impact would be.
We’ve now got to the end of the billing month and, while our data usage has increased by 12GB overall (that is, an average of 160MB per iPad/iPhone running iOS 9) - the feature doesn’t seem to have impacted us at all. The higher users - most of which have upgraded to iOS 9 (as have 57% of our users by the way) are all typically high users anyway (all of whom have been contacted…)
In a nutshell, the feature looks at your WiFi connection and, if there is little or no internet connectivity available, the iPhone will automatically use cellular data (essentially so you should not need to disable wifi just to be able to do whatever it is you are trying to do).
Obviously, the phone will still adhere to whatever settings you have as to which apps can and cannot use cellular data. It won’t try to do anything silly like iCloud backups or software updates over cellular (unless you’ve turned that facility on of course).
The official detail from Apple on how this feature works can be found here: https://support.apple.com/en-us/HT205296
In summary, use the feature - or not - it’s up to you.
The main thing is to keep an eye on what you are trying to do over data - basic rule of thumb is that watching video or using streaming audio services over cellular data is not a good idea.
Most of all, don’t panic.