I'm of the opinion that any sort of generic power supply is going straight in the bin. Most of the failures I've seen are just melting or the DC side cap blowing up, but I have absolutely no desire to be on the end of a transformer insulation failure. You're literally trusting this stuff with your life, it's not the place to skimp on the price. Apple chargers are cheap and efficient for 5v USB stuff, for the rest there's cheap switching supplies on Mouser that actually have semblance of safety regulations being adhered to.
If you want to get these things for free, go to a hotel and ask if they have some left behind that you can have.