Nomex is a fire retardant material that is mainly used for flightsuits and aircrew wear. It is also used in firefighter gear and has become popular with close combat and high risk entry teams such as SWAT due to the possiblity of fire in these environments. As nightgaunt mentioned they are hot and itchy because they are 100% synthetic. You don't need Nomex unless you deal in incendiarys or risky fire situations. Plain Propper BDU's are great, I like the ripstop 50/50's the best for all around wear and cotton ripstops for hot weather wear. I don't like the 511's, they don't have flapped rear pockets they have odd pleats, they are kind of heavy and look way too "authoritarian".
Tad