FYI - Harbor Freight sells a similar gadget for around $20. The reviews are generally good.
As an electrical engineer, I understand the theory of how it works. I also understand there are circumstances that could cause it to not work that have more to do with the wiring of the electrical system where it is being used than with the design of the gadget.
If I were a commercial electrician who wired houses for a living, I could see how this might save a few steps, and perhaps a little time. But as a homeowner, there are equally good alternatives that don't cost anything, and I have better uses for the $25 (or $20 for the HF version).