I use them. Yesterday this machine shut down in the middle of web browsing, the 550W supply lost the 12V side (which the tester showed). They are good for testing all the power plugs without risking a working board or expensive video card. Most power supplies have a PS good signal (self diagnostic) and the testers should have a LED for that.



24 20 Pin PSU ATX BTX ITX SATA PC Computer Desktop Power Supply Tester | eBay

For the price I don't see why everyone doesn't have one. Of course you have to make sure the buyer of the supply knows not to use a 250W power supply on a new gamer system that needs a 700W one.