Beware: you can unknowingly burn through your cellular data if not careful. With some apps you may want to use on both wifi and cellular, but use it differently on each mode to manage data usage. Specifically, I'm referring to apps for chatting, where you can choose whether or not to use video. If I am on cellular, I will only use voice. If I'm on wifi, I'll use video. However, with wifi assist, if you are using video because you are on wifi, but then your wifi drops off, you will then be using video on cellular and, therefore, unknowingly burning through your cellular data very rapidly. I unknowingly burned through my entire month's of data just chatting with my girlfriend.
There may be other kinds of apps where this may be an issue. I do wish Apple had either turned wifi assist off by default or otherwise let me know this happened. A warning indicating when you've switched to 3G would also be helpful. (As an aside, I know some apps distinguish between cellular and wifi; it would be great if, for example, Google hangouts had an option to disable video when on cellular...).
Cheers!
