BFL's legitimacy is dead simple to assert: they managed to be greatly successful and profitable inventing, making, and selling thousands of FPGA Singles. I have many of them in my very own hands.
And if you have a profitable business, the natural human reaction, even for greedy people (the type who may want to defraud), is to simply expand the business, not to do something riskier by defrauding your customers.
Pirate paid out for awhile, too.
The difference being that Pirate was obviously loosing money by loaning it from his customers at 7%/week.
Whereas BFL is obviously making money by selling the FPGA Singles ($430 or $150 profit margin out of the $600 price, as per FPGA cost estimated by
ngzhang to be respectively either $50 or $200, plus $50 for the PCB/case).