Actually FPGAs had 10× better energy efficiency. This was huge. Eg. the popular Spartan6 LX150 used in many miners was rated 50 joule per gigahash, compared to

Actually FPGAs had 10× better energy efficiency. This was huge. Eg. the popular Spartan6 LX150 used in many miners was rated 50 joule per gigahash, compared to 500 J/GH for an HD 5970. I used to run both GPU and FPGA mining operations. That said, they were comparable to GPUs in terms of $/GH.