Interestingly,
a study was done in 2003 of actual use. Turns out many a-holes just ignore the rules as proved by the on-board spectrum analyzers they flew.The opinion of the engineers was that cell phone use, at least, did represent a real hazard.
I know that if I am on a flight and see someone using a phone, I will reach for the call button as fast as my hand can move. What really needs to happen is the FCC needs to evaluate all gadgets for their air safety as part of the already required FCC certification and certify and label them as to whether they can be used in flight or not. So far this has not been done.