I have a Western Arms MEU/SOC, and as several folks who were at Cold Burn can attest, it performs very well out of the box. On the chrono it shot a consistent 310fps with green cas, and the performance held during rapid fire.
The build material is, as busta_cap says, a very hard composite. On the MEU the weight is added by using grip scales made of metal and covered with a tough, rubber coating on the outside. All levers work as they do in the original.
I haven't shot the KJW guns, but I have to say I don't for a moment regret the extra money I spent on the MEU.
-Chris