I wouldn't tattle on anyone who continued using a mobile phone or any other electronic device,
A. Because I'm not about to start an argument with a fellow passenger.
B. Because I'm not convinced by the claims that electronic devices can interfere with the aircraft.

However, I would probably give dirty looks to someone who continued talking on a cell phone while in the aircraft. To me, aircraft safety aside, that's just plain rude.

I was recently asked by a FA to switch off my e-reading device prior to take-off. It is not one that can either transmit or receive. It's just a storage device for books that I have downloaded on my computer and then transferred to the e-reader.

Since my particular e-reader doesn't send or receive anything, it seems to me that switching it on would have no more effect on an aircraft than switching on a torch/flashlight.

Perhaps someone with more technical knowledge than I can either defend or contradict my reasoning?