Nitrogen is more thermally stable. ie. hot and cold pressures don't vary much.

Nitrogen also leaks less, so you don't need to pump 'em up as often.
(mind you, I've had my tyres filled once, at purchase, over a year ago, they hold their pressure just fine).

edit: nitrogen makes up 70% of normal 'air'. but if you can get it free with a new set of tyres, why not get it.

Air contains moisture which expands when the tires get hot so you do not have a constant tire pressure, very noticable on a long highway run or a hard drive threw the hills.

Nitrogen does not, so stays at a constant pressure

kool might have to get nitrogen in my tyres at my next wheel allignment

thanks guys