Thankfully, my job hasnt taken me to Iraq (closest I've been is Turkey), but some co-workers have returned with stories and I've had the chance to shoot the breeze with some military men that engage in both frontline and clandestine work.
And I have to say, there is lots of negative feedback on issue weapons. Mind you, the weapons in the field have seen hard times, abuse and live action worth thousands of rounds. Subject any weapon to enough abuse and it will fail regardless of who makes it.
My co-workers, though, have the same type of weapon (RRA or Bushmaster or something custom usually) in the same conditions and they never complained. Mind you, the company's guns ride in locked and padded cases, and are seldom fired in self defense. When the day is over for us we retire to a hotel usually. I'm sure if I lived in battle conditions 24/7 my gun would be guilty of a few quirks as well.
The one thing I would change about my weapon is the fouling, which is more of a potential problem than an actual problem. Mostly I carry a PDW anyway and it's concealed 90% of the time