I read a lot about the Haramaki. It sounded too good to be true but it also sounded as though it could actually work! Before I spent my money I walked about with a cashmere scarf round my body and yes it did seem to keep me warm but kept falling off! In the end I gave in and bought one (soon to be two! I don't think it's good for it to be dried out on a radiator!)
There is a lot written about it, main thing is it keeps your "core" warm which means the rest of you keeps warm.
I dread the winter and this has made a real difference. You can wear it next to your skin, over your knickers...whatever. I wore it over the whole lot yesterday as I forgot to put it on and didn't want to get undressed again. Seems it's quite hip to do that too lol! It still worked.
I'm sure it would be just as good for the gentlemen as well. After all it was the Samurai Warrior who started it off!
As usual...read and learn and make your own minds up. They are not cheap, about £20, but they really do work