I own one, I've shot one, and all I can tell you is my personal experience. I shot some really old .22 longrifle ammo through a very dirty GSG-5 owned by a friend of mine. We shot probably 150+ rounds through it, all at rapid fire. The GSG shot and fed perfectly, and at 10 meters, I was able to put all 20 rounds I was loading into each magazine into a ragged, quarter-sized hole with a cheap little red dot sight.
I was so impressed I immediately went out and bought one for myself.
I've posted another couple of threads about my GSG, you can find them in here.