I've been wearing BDU pants since they were first issued back in the 80s.
I was issued OD cotton fatigues in the Air Force and wore them the whole time I was in the service - They were cheaper than the BDUs and sharper looking - But I sure as heck purchased the early nylon/cotton trousers and used them for all my outdoor adventures. I've worn 'em weeks on end without washing 'em, letting them dry out on me while I'm wearing 'em if they get wet - As have a great many G.I.s over the years.
Some of the new BDUs are 65% poly and 35% cotton, and are even better than the old 50% nylon 50% cotton type.
Best of all, I can still find good condition used BDUs at local surplus stores for about 12 bucks!
Combined with a poly/cotton dress shirt, this outfit has become my standard for most three season weather.
They are heavy though, at about 26 ounces. Still, backpacking, canoeing or cycle touring, that's what I wear.
Edit - Uh, I'm even wearing clean used BDU trousers and a dress shirt here at work today!