Nitrogen makes sense in a race/track car, aircraft or heavy industrial use. For the rest of us, not so much. The advantage is less moisture vapor (about 1/2% of air, with a decent moisture trap). The nitrogen separators trap most of that water and fill the tire to about 94% nitrogen. Consider, air is 78% nitrogen. As air escapes the tire, about 75% is oxygen and 25% nitrogen. Each time you refill, you are replacing with 78% nitrogen, 11% oxygen. Gradually the nitrogen mix is getting richer w/o spending money. The bad: Moisture capture systems at tire shops are getting better, but will never remove as much water as the Nitrogen generators, but air can hold moisture, nitrogen apparently cannot.
The cost savings mostly come about because the tires remain at a constant pressure. You have to add air about 1/3 as often, from air loss. With reasonably accurate TPMS sensors, we can tell when air pressure drops below normal, so running improper pressure isn't the problem it once was. With tire rotations, I'm having to drop/raise pressure every 5K miles in any case. The TPMS system lets me know when pressure varies as air temps change. I was in the driveway last week, adding 3 pounds to all 8 tires on my cars. Another advantage... exercise from using a hand pump. <g>