Looks like a lot of U.S. design ideas were copied....
The Su-27 is an indigenous Soviet design, and is worthy of respect, if for no other reason than the fact that the Soviet engineers managed to create a craft with near-US-fighter capabilities (the Su-27 is probably the world's most agile aircraft, despite being damn big, google Pugachev's Cobra) using 1950s technology. The Chinese picked a good base to start from, but as they lack the massive aerospace program the Soviets built, they will have a hard time improving much on the design anytime soon. That being said, this thing is SO raptorbait.
Cheaper and easier than doing it themselves.
The ugly underside to this story is that the Chinese industrial capabilities that make this possible, are the very same ones that get outsourced from the US.
There's always been a national security aspect to outsourcing ... and here it is.
No doubt. And, all the US quality management and high tech development principles have been learned by all the Chinese engineers and managers who used to work for Western companies and their Chinese partners. They then change jobs into defense and there you have it. Offshore a server or chip today and get a trained Chinese defense employee tomorrow.