A couple of things that I think about when buying knock-off parts:
If it's a functional part and quality will be an issue when compromised, such as bearings, brakes (levers is as debatable as it is being here) and engine parts, I'll stick with original respected brands. With the exception as someone mentioned here is if they specify industry standard and material (ISO9999 100% titanium), then you'll probably be ok.
Parts such as carbon bits and oil caps, knock-offs nowadays should be ok.
What bothers me is if someone in parts of the world can make a $1000 part for $100 and make money, then these parts are overpriced in the first place. Do the raw materials on a rearset with considerable payoffs to the engineers and designers worth $1000? This doesn't matter to people who drive Bugattis to pick-up eggs, but for me, I'd rather spend my hard earned cash on something more significant.
If a company invests 90% of their money on something that can be copied fairly easily, they won't get very far in this day and age. A lot of things are made in China and other parts of Asia now whether you like it or not, and might as well be on the long end of the stick !