It is a great tool. Like Riptor said, taking out the need for row 6 (ep)ROMs to be functional is very handy but the pac board has to be somewhat functioning for the diagnostic board to be of any use. It also will help troubleshoot the watchdog circuit, color rams, etc. But like any tool, its usefulness depends upon the experience and knowledge of the user. It's one of many you will need in your repair arsenal. If the board has interconnect issues or is dead as many do and are, then you will need additional tools like an oscilloscope, logic probe, etc. to troubleshoot those issues first.
Bill