What I've read (FWIW) is that cell phones can cause issues during takeoff and landing. Beyond that, the biggest issue seems to be draining your battery while your phone hunts for a cell network on a flight.

I'm really happy about this because I like to catch up on reading email and drafting responses (even if I don't send them). Losing 20-30 minutes each way on both takeoff and landing (total of almost an hour) is a pain. I don't read much of anything anymore that is in hard copy format.