Using Nitrogen in tires is one of the new trends on the block. I tend to doubt that it makes that big of a difference, but here is the low down.
Nitrogen molecules are larger than the other gasses naturally occuring in the atmosphere. (By the way, normal air is already 80% Nitrogen)
Tires will not deflate because the larger molecules can not penetrate the rubber tires as easily.
Since the Nitrogen was from a compressed source you will not get as much water (humidity) into the tires.
Due to the lower water content, and lack of Oxygen inside the tires, your tires should suffer less decay to the rubber and belts and last longer. IE: normally Oxygen and water passing through the rubber tires tends to cause it to slowly break down and can cause steel belts to rust.
They claim that if you have the tires filled with Nitrogen when new, but still top off with normal air, you will still get most of the benefits.
In my opinion, I would not pay extra for this, but if I had the choice I would fill new tires with Nitrogen just to get the benfit of them maintaining their pressure better. As for them lasting longer, I wear out my tires much faster than the rubber decays. If you only drive 1000 miles a year, and plan on keeping the same tires on for 10 years, then maybe that is a consideration for you.
Craig
PS - They have been filling Aircraft tires with Nitrogen for years, so this is not really new.