Nitrogen filled tires... is it worth the money?

Following routine maintenance on my wife's 2009 Versa SL, my local Nissan dealer offered to fill her car's tires with Nitrogen. They indicated that there was a one-time $30 fee and that they will top it off as needed.

My question is as follows:

What do Nitrogen inflated tires give her more over regular compressed air?

I suspect that the nature of Nitrogen vs. air would mean that the tires would maintain steady pressure during city and highway speeds (less affected by heat). That she would see less difference between cold and hot tires.

Essentially, is is worth the money and the effort?

You pretty much answered your own question; Nitrogen will keep constant pressure on the tires (given that you don't have a slow leak) and improve tire life. Also, tire temperature will be lower(slightly) reducing tire wear and prolonging the tire service life. Still, 30 bucks is a lot money and add to that the drive to the dealership to get your tires inflated I would much rather put that money in buying a compressor and inflating my tires at home.

I used nitrogen in our circle track race car. This gas is more stable due to the low moisture content. The idea is that the H2o will expand increasing the tire size when the tires get hot. With compressed air the tires would grow as much as 2" changing the stagger.
On a conventional tire on a street car the likelyhood of this happening are very slim and unnoticable. Unless you plan on racing the car it seems to be more of a gimmick than the networth results. Check your tire pressures cold and hot and decide if there is a great difference in pressure that would warrant the expense.