Using pure Nitrogen in tyres is supposed to reduce pressure changes due to tyre temperature and reduce leaks as Nitrogen molecules are larger than Oxygen molecules therefore maintaining tyre integrity.
TBH it's probably a load of cobblers but does sound cool.
Saying that F1 and aircraft manufacturers have been using Nitrogen for years so who's to say (although those tyres are meant to take a lot more abuse heat wise).
It was £1.50 per tyre at Kwik-Fit with free top-up's as needed- I know, not the best bunch around but when you have two tyres going down and the tyre place is at the end of your road what are you to do?
Personally I feel the wheels seem more stable but could just be due to two new Goodyears on the car and the fact my tracking and balancing was way off.
All in all I think it's personal choice.