They do use less energy. A 35 watt HID pumps out a shitload more light than a 100W halogen / xenon.
Some of them 60/55 +50 whatever they call them bulbs are pretty sweet, if your car takes H4 hi/lows. Ya probably can't safely run 100W without upgrading the wiring and bulb housing, as they pump out a lot of heat and pull many more amps, but 100W highbeams are quite lush.