I bought one of these from an electronics company via the amazon marketplace and it turned out to be a FAKE memory stick, I put it in my psp and it rejected it, i attached it to my computer and it denied that it was a memory stick.
I wouldn't mind if it was counterfeit and WORKED, but it doesn't. All i am saying is be careful because this could happen to you, so i'd recommend using a trusted seller, for example amazon.co.uk rather than the marketplace.
According to Ebay, 98% of the memory stick duos sold on their site are counterfeit. Ebay have a good guide entitled "FAKE Sony Memory Stick Pro Duo MagicGate Cards Exposed" on how to spot a fake.
Be careful.