So the thing with G&G is that unlike most airsoft companies the quality of their stuff varies a LOT based on what product line you are buying.
Combat Machine's are their bottom of the line, these are not good, they tend to break easily and tend to be problematic overall.
MAX guns are pretty decent, usually metal upper and polymer lower receiver (or vise versa) and I've heard pretty good things about them in general.
Top Tech is their top of the line, these guns are solid and compare to many of the good airsoft guns out there, they are full metal and somewhat upgraded already from the factory.
Personally I don't really like G&G, if I did buy one it would have to be a top tech.