I have used them and they work. I left my headlights on in my '69 1/2 ton for 8 hours. I hooked up the magic box and left it on for about 5 minutes I guess and it started!
I have found that you need to leave them on for a while before you try to start the engine.
They are kind of like chiropractic care and acupuncture. Don't ask how they work. They just do.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission