Cooler air is generally great for helping to get a little more power. The usefulness of cold air intakes for better gas mileage is debatable.
Essentially a cold air intake gets denser air (more oxygen = more fuel = more power)
A hot air intake does the opposite (less oxygen = less fuel = less power)
With hot air, you have to open the throttle more to get the same amount of power, so you reduce pumping losses in the engine and make it more efficient. (that's the theory anyway) Only way to tell for sure is to try it
