Dimmers are a simple and nice update to a home. They allow you to adjust the lighting to fit the situaion.
Dimming the lights does make them last longer and you use less electricity. But you do loose a little efficiency (2%) at the outset. A fixture without a dimmer is slightly brighter than a fixture with a dimmer at full On.
Dimmers have rated capacities, usually 600 watts. (You add up the wattage of all the bulbs controlled by the dimmer.) Special dimmers can handler 1000 watts and 1500 watts. If dimmers are put side by side, they must be derated so they don't overheat: 600 watts down to 525 watts, or sometimes 450 watts.
It's normal for the dimmer to get warm while in use. (2% of the electricity is lost to heat in dimmers) Go to this link for more information from Lutron.
Helpful Hint: A warm dimmer is normal, but it should not get so hot that you get burned when you touch it. A metal switch plate helps dissipate the heat evenly, a plastic plate's metal screws will get extra hot as they conduct the heat away.