I got a pair of CX300's off of eBay a while ago. The seller claimed to be in the UK. Later it became clear that they were being shipped from China and was faking his UK status. I have been to China recently and seen with my own eyes that virtually the entire retail sector in China proudly offer counterfeit merchandise. Needless to say, I would never have ordered them had I known the seller was Chinese.
That said, the ones I got were either really good fakes, or the real thing. Of course I don't actually have a way of knowing, as I am sure good quality fakes do exist. I am also sure that different fakes exists, so that looking for a particular physical attribute that will give the fakes away is pointless.
As for the sound quality of my (real or fake) CX-300's, I found them adequate for use in the gym. They're bassy, real bassy, without too much detail. They're definitely a step up from the ones that came with your mp3 player, but not that great either. At the price (it seems about $70 will buy you a pair from an authorized dealer), you can do much better. Eighty bux at Staples (!) for a pair of JBL 220 is a bargain by comparison.