I have heard about this from several different people. Like everything, there are pros and cons, the pros being: the nitrogen reduces or stops the need to keep adding air to your tire, the tire gets better tread wear, keeps the tire more rounded, prevents the tire from getting a flat spot while sitting for long periods of time, and performs better. The cons being: it keeps the tire temperature down, which reduces the "sticky" effect to the road when you ride, may give a harder ride if you tend to run your tires a little on the low side, and the air we breathe already is about 78% nitrgoen, 21% oxygen, and 1% miscellaneous gasses.
For me, I do not see the value, but if others do, great!!!