Good luck figuring out why your code doesn't work on your "genuine" 8051.
From my experience, as soon as there is some demand (this has hit HN after all) the resellers start selling fake or inferior chips. Doesn't really matter how common or cheap they are, its just hope things work now.
Edit: when we really have to buy from China, I always create a test rig to verify component characteristics. Easy for transistors and opamps, not so easy for CPUs.