We used one a few years ago when everyone was getting upset about EMF from power lines. Once most of the arguments had been debunked, it kinda fell out of use.
Obviously, you'll get higher readings around running appliances than you will around, say, the wire in the wall.
And if I remember correctly, the strength of the field is inversly proportional to the square (or cube?) of the distance to the source of the EMF. So someone would be much more exposed to something up close like an electric blanket, than they would their breaker panel.
As for cell phones, I would think the RF frequencies would be more of a worry than the EMF.
