As I understand it, the Nitrogen is less reactive to the other suspension components thus more stable in a compressed state. I'm no chemist, so I may be way off base.
However, the pressure is pretty high and thus a rebuild can only be done at the factory.
At least that's what Rock Shox told me when I had to send in my Coupe Deluxe for a rebuild!
I switched to a Fox Vanilla RC and haven't had any problems.
I am considering switching to a Progressive, but have heard about reliability issues.
L8R
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger