Then you must know what is the original, and in the case of the Peak C-5, it isn´t as simple as some people seem to think. I´m basing this on that more and more often I see people stating that this and that is a copy of the Iwata similar model, when Iwata isn´t more the original than anything else. The only real original Iwata design that I know of, is the Eclipse (which in my honest opinion is a fabulous airbrush).
I don´t want to restart the "which airbrush was first" war, but to sum it up, all of todays airbrushes are more or less copies of older designs. There´s not much happening in airbrush development.
So, to me, the use of "knock off" is more of a degrading term. It´s implying that it is copied from something better or original. In general, this might be correct when it comes to a lot of the chinese made airbrushes. Not that they all are bad, but they really don´t hold up to the japanese, european or american brands. But they don´t cost as much either. And the main difference is in things that can be corrected or enhanced. The needles need polishing (or even better replacment), and the chrome isn´t as good as in more expensive brands. One of the things that I feel immediately is the creepy feel of the main spring, and the usually rough feel of the trigger. But with a polished needle and trigger parts, a new spring and maybe a new preset back handle, they can perform just as good as any other airbrush.
Do I recommend the Fengda airbrushes to beginners? Usually not. Why? Mostly because there are enough possible pitfalls in the beginning stage of airbrushing anyway, and you really don´t need to be unsure if the equipment is working with or against you.