I always thought that dimmer switches were great - you can make the room bright when you need it bright and dim when you want it dim.
I always thought that dimming down the lights saved energy. It does... but apparently not quite as much as I thought. It's not a linear relationship. If you take a 60 W incandescent bulb and dim it to look the same as a 40W incandescent bulb, it still uses more power than a 40W incandescent bulb.
The old dimmer switches were especially bad. They used a variable resistor to essentially eat some of the voltage and thus drop the voltage across the light bulb. Depending on how you turned the knob it would eat more or less voltage. A variable resistor eats voltage by dissipating it as heat. So while the bulb was less bright and using less power, the switch was using up most of it. The numbers I have seen suggest that by dimming a light 50%, the power usage would have only dropped 20%.
But that's the old stuff. Instead of messing with the voltage across the bulb, triac-based switches turn the current on and off really fast, essentially having the bulb on less often. Theoretically this happens faster than the eye can detect. By taking this approach less power is wasted because instead of pushing it off on a resistor the current is actually being shut off for a short amount of time. The more dim you set the switch, the less time the bulb is on for. There are still some losses but not as much. The numbers I have seen suggest that by dimming a light 50%, the power usage would now have dropped 40%.
So by using a dimmer switch you are still losing approximately 10% or more of the power.
Maybe in some cases it's worth the experience. Maybe in others it's not.