I have an old USAF Nomex flight suit (CWU-27), and besides the advantages listed by Genghis, have also noticed that it keeps me warm in cold weather without making me sweat when it's hot, and it's also dirt-repellent - I remember helping someone drag rusty and muddy pipes to clear up some extra space on the airfield, making a mental note to take the suit home for washing, and then seeing no trace of that rust and dirt on it a couple of hours later.