In the meantime many people have build the GALBlast circuit and most have succeeded. But some problems have been reported. Most of the problems are self made by using different parts than mentioned or trying to use a bread board layout, but some still remain:
Many people had problems ordering the ZN428 used in the last design. This has been answered by using the AD/MX/TLC7524 and LM335Z2.5 in this version.
Other people had problems ordering the LM78S40 or the required 470uH coil. Now there are more ways to power the circuit. You don't need to build the switching regulator if this sounds too complicated for you.
CUPL4.8 has been reported to generate JEDEC files for the 20V8 that contain the CFG bits in reverse order. I'm not sure if this is true.
ISP Synario often reassignes pins even if an assignment of signals to pins has been defined in the .ABL file. Have a look into the report files.
Some people would like to have zero insertion force sockets. Buy a single 28 pin skinny dip ZIF socket and insert this into the desired socket on the board. Make sure that the socket soldered in is suitable to handle a ZIF socket.