does WiFi Assist of iOS 9 work when cellular data is turned off? Cellular connection kicks in when the WiFi is weak, that's a nice feature in iOS 9: WiFi Assist.

I have on question, when WiFi Assist is ON. Does it cause me cellular data in the following scenario:


WiFi Assist is ON
Settings > App and iTunes Stores > Use Mobile Data is OFF
Now, WiFi is good
I am downloading a huge app
... so far so good
suddenly WiFi goes weak ..


Does AppStore continues to download and consumes my cellular data, or just stops?
In your case the off switch for mobile data will prevent the use of data across the board. 

You won't get any benefit from turning on "assist" nor will you incur any downside or costs due to data usage.