Owned my G&P M16A1 for quite a few years now. It's not fielded super heavily, but it is my mainstay gun (I just don't get to play as often as I like).
I've dropped it once or twice, crawled through trenches, played in the pouring rain, with different batteries and other mild abuse. It's still holding up. My hop up seems fucked now, which I may look into, but that may have been operator error (I've forgotten to raise the hop all the way before cleaning). It's just a shame the Colt Trademark ones are no longer widely available.
It's the most historically accurate Nam-era M16 I could find, especially if you can get the Colt/US govt trades. It only has 3 major inaccuracies:
Light fins on the end of the barrel
Selector engravings on the right and left side of the reciever
laser-etched "Made in Hong Kong" on the left side (I covered that with electrical tape).
Other than those, it's very good. They even got the proper front sight post (round, not square). G&P (which uses clockwise threading for muzzle devices) is also the only airsoft company I've found that makes the proper A1 birdcage (ports all the way around, farther apart than the A2).
Some other notes: The included handguard is quite cheesy. I replaced mine with a surplus Vietnam set, and it looks great. The end cap for the handguards is loose on mine, too, but it doesn't affect anything. The stock is also not quite big enough for a 9.6 NiMH. The A1 had a shorter stock than the A2, and G&P represented that, for better or worse (better for me, I prefer shorter stocks and historical accuracy).
And don't forget your OD canvas M1 sling, and replica M7 Bayonet.