I have a jg mp5 navy (metal). Bought it last summer. Immediately upgraded the spring guide, cylinder, piston, piston head, shims, grease. Gun has been running flawlessly for around 15 games/25,000 bbs.
Great gun. But I have seen some that weren't as nice out of the box.
Have an echo1 mp5k pdw as well. Needed new piston head out of the box, grease was shit (as usual) and the battery is garbage. It has run well though since last summer
Grand Poobah of T.W.A.T.