ive got some scratches on the fake ones but i had a genuine pair and they also got scratches and i had to replace them.. I find that the fake ones go on the screens really easily but the genuine ones are harder to get on.. I have a Blue DS Phat and sometimes i can only get the DS Lite screen protectors but they do the same job, although top screen on DS phat is bigger than that of on the DS lites..
I also bought a pair of genuine DS screen protectors from EB Games, asked for the DS phat screen ones, bought them, took them home and opened it up to find that they were DS lite ones.. Took them back, asked for DS phat ones and they said they were the same, so i tried them on my DS on they turned out to be a bit smaller for the top screen.. So i couldnt take them back after i used them, so i thought that they had conned me into getting them..
Anyway, back on topic... The fake ones may not be as long lasting as the genuine ones but do the same job.. I think that if they're cheap on Ebay, they're bound to be fake, if they're around 12 bucks a pair, (Aussie Dollar) They're genuine.. This may not always be the case.. Most sellers that sell genuine pairs will usually say They're Genuine, but on the fake ones they dont.. Although this may not always be the case because the fake sellers will lie to sell their products..
Usually, and Im not being racist here, the fake Hori screen protectors will come from Hong-Kong, and have a low Buy it Now price on it.. They will also usually sell them in sets of around 5-10 and will not say that their products are genuine.. This may not be true to some of you GBATempers but, Im sad to say this, it is..
If you think I am being racist, sorry, Im merely stating my opinion and am not meaning to be in a racist tone