Today's portable electric heaters are relatively safe compared to liquid fuel or open flame heaters, but utility companies appear to disagree about the cost of that comfort. What makes an electric portable heater useful is its ability to quickly warm an area or object.
Convection heaters warm the air and are best at heating enclosed, relatively draft-free spaces.
Radiant heaters warm people and objects directly and are better suited for garages, drafty spaces and larger or more open areas. In either case, what makes portable electric heaters viable is that you can turn them on until the area warms and then turn it off. Newer models with thermostats work the controls for you. Crank up the furnace to take the chill off you in one room and you'll spend much more because you'll unnecessarily heat your entire home. The key to using portable electric heaters economically is knowing how much it costs to operate.