This thing has saved me so many hours of troubleshooting. I can't count the number of times I've walked into a venue and had the local techs tell me all the channels on the multi are good, yet somehow one of my mics sounds off or a DI isn't powering!
Rather then testing everything in the signal chain, I can just turn on 48v, whip off the microphone and stick the Male end on the end of the XLR. Three green lights, and I have a dodgy microphone. Anything else, and the cable is bad. Follow it back down the chain until you isolate the fault, and then bypass the problem cable!