Here’s a cool LED HOWTO guide including how to power LEDs using DC or AC.
LED lights are extremely efficient compared to standard incandescent lighting. No other lighting source outputs as many lumens per watt. They are particularly efficient at producing a single color of light. Other light sources have to produce the entire spectrum and optically filter out unwanted colors. However, LEDs can be manufactured to produce only one wavelength of light. This makes them particularly useful in stop lights.
Another advantage to LEDs is their operating temperature. Most available today can be function for hours and remain cool to the touch. Directionality is another key property. They only emit light over a relatively small angle. This can be advantageous for reading lights, but hinders performance when attempting to light an entire room.
If the advantages of LED lighting interest you, Myths Busted, LED Lighting is an excellent article by a researcher in outdoor lighting solutions. (And the source of most my information.)