A note for those using slime in their tires. It is fine on tires that use tubes, but if you really want to piss off your local tire dealer, use it on tubeless tires. It makes a real mess on the wheel and difficult to work on.
The reasons for using nitrogen to fill tires is that it doesn't migrate through rubber as fast as air, it expands and contracts less than air does when the tire temperature changes, and it contains less water. http://www.popularmechanics.com/cars/how-to/a3894/4302788/
