This could be handy if you are in the system building business. Best, of course, to buy high-quality power supplies; you'll have much less worry about testing them!

Would this tell me anything useful? Knowing I've got 12V +/- 1V on the 12V rail is hardly news. Its not an occiliscope or a precision DVM.

I could be wrong, but it seems to me that this is a DC Power Supply Tester.

AC, it is not DC

like the hermit said, will this tell me anything that i really need to know of use?

The type of switching power supplies used in PCs require a small load to regulate properly, so you can't bench-test a power supply without something like an old motherboard to plug it into. You can then check the voltages with a $3.99 multimeter.

This provides a very light load for proper regulation and a simple go/no go test of the various voltages. It's just a time-saver for quick and dirty testing of a questionable power supply. Nothing special.

And yes, it's strictly DC voltages only.

This does not even provide the minimal load most (practically all) PSU manufacturers spec as required for proper regulation.

What it can do is find some kinds of PSU faults that prevent the PSU from running at all or if it were running way outside of spec even without a load.

What it cannot do is properly load the PSu in any meaningful way to determine it works properly, it cannot qualify a PSU as working when that was the very thing you hoped to do, determine if the PSU is ok.

This mostly makes it a waste of money, though you could help improve the situation a bit if you had half a dozen HDDs plugged into the PSU as a load but even then it can't qualify the PSU for a particular system's use, and if the PSU operation is in question you don't really want to plug it into that (valuable?) system in case it's spitting out crazy high voltage.

In short, if this were sufficient then even lazy web reviewers would incorporate it into their review. Better to buy a $20 multimeter, as even though a $5 cheapie would suffice in accuracy it will tend to be short lived junk with short fragile leads and often using a less common and shorter lived, more expensive 12V battery. Multimeters that use 2 x AA are very desirable for long term ownership.