I'll differ slightly from the mostly correct opinions of the others - depending on how your OEM air intake is designed, heat soak might be a problem - that's when you sit at a stop light, underhood temperatures rise, and you suck in that warm air when you take off. Warm air is less dense than cold air, and so you lose power. so, a CAI will help there because it's sucking in ambient-temperature air rather than hot underhood air.
That said, if you're worried about drag racing at stoplights, your priorities are screwed up and you need to stop.
Otherwise, they're a waste, and are sometimes potentially damaging - some CAI's go below the bumper, and if you drive through a puddle will suck up water and damage or destroy the engine.