I gather about 95 per cent of the power consumed by an incandescent buld is emitted as heat. I'm sitting on a chilly night in a room with the only source of heat, aside from the PC, whic is pretty cool, is the bulb in the desk lamp, and it's quite comfortable.

I suspect it's a more energy efficent way of staying warm at a desk than putting the central heating on and warming the whole room - more localised.