I own the WE M4A1 AEG. Have ran thousands of rounds through it and gamed it all weekend at Rhino putting about 4k through it. For the most part its been very solid. I had minor issues with the wiring in the grip where the positive feed was pinched from the manufacturer. It caused the gun to overheat and blow the fuse. It was stripped down, the fuse was replaced and the motor reinstalled and has run fine since. The wiring in the grip us finicky and requires a lot of patience to get just right so the bottom plate will go on, but its a minor headache. I upgraded the barrell to a tightbore Madbull 6.03 and a metal hopup ($65) but the gearbox itself has been perfect. I run G&P 9.6 1300MaH and it pumps out at a reasonable rate of fire and decent fps.
Don't get me wrong its not the best gun on the market but for $300ish its a solid choice.