I own two KA M4s and I love them externally. Internally you're still playing the Airsoft lottery, but the odds of winning are fair. Also, as mentioned earlier, if you do lose, there's a ton of aftermarket parts to fix it.
While I've heard good things about the aforementioned G&G CM16, keep in mind it has a plastic body. I personally think a (decent) metal body is a must, but if you don't mind the plastic, go ahead.
K3vX | Airsoft Gunsmith/Gun doctor in training.
Feel free to contact me in your preferred language and I'll gladly answer.