I have the Cansoft version for awhile now and although I'm not an Airsoft veteran, I am extremely happy with my purchase.
I have only shot about 1000+ rounds or so, but I have never experienced any problems whatsoever. On full Auto or quick bursts with Semi, the magazine does cool down considerably fast. However, it does not seem to affect the way the gun shoots. I have tried using Duster and Propane and the recoil has always been strong, and there hasn't been any problems with having the bolt lock back.
In all, I love the way GBBR's add to the realism, and the KJW M4 has been very impressive. I would highly recommend this gun.
P.S. Does anyone know which Metal Lower Receivers are compatible with this GBBR?