At my dealership we put nitrogen in the tires of all of our cars. I can't say anything with regards to seepage, but I have noticed that nitrogen does result in a lot less "low tire" warnings once cold weather hits.
Ditto at Don Davis Nissan -- when I bought my '09 coupe they mentioned nitrogen in the tires. Something about being lighter and overall better. I still got a TPMS warning light once or twice when the weather changed over night but nothing repetitive. First and last car with TMPS + Nitrogen that I'm aware of, so I can't say if it was better or not in any aspect.