Chris Mills, writing for Gizmodo:

Amongst its big bag of tricks, iOS 9 has one seemingly innocuous feature: Wi-Fi Assist, enabled by default, which will switch to cellular data when your Wi-Fi sucks. This would be great, if I had an unlimited data plan. I don’t, but now I do have a very expensive cellphone bill.

This is shocking to me. While this may be a truly useful feature, why would Wi-Fi Assist be enabled by default? Any setting that automatically consumes my cellular data (and therefore spends my hard earned cash) should be OFF by default.

To check your Wi-Fi Assist setting, go to Settings > Cellular (Settings > Mobile Data in the UK), then scroll all the way to the bottom.

Please pass this along, especially if you know someone at Apple who might be able to get this setting turned off as a default. And if I have this wrong, please let me know.