They are quieter. I really notice a difference at about 100'.
I got taken as well. Bought a set of cheap knock-offs. They did work for a while but eventually they slightly loosened up at the hub hinge and then I was getting a motor error message when I started up my Mavic, the motors would immediately shut down. I put the stock props back on but did order genuine OEM blades, no issues since.
I did notice the knock-offs were very similar but the rivets that hold the blades to the hub has serrations where they were flattened out that seemed abnormal. The blades seemed identical and also carried the same part numbers.
I think that these blades were rejects from the factory, removed from a scrap bin and then resold by some enterprising individual. As the old saying says "Caveat emptor."