I use one sometimes, banded just beneath my bra, with the gun's handle between the bra cups. For a woman with some curves, it can be an excellent way to conceal a gun while wearing shorts and a t-shirt, as long as you are careful to wear a stretchy t-shirt that allows you to draw the gun quickly when you need to. (A boat neck or v-neck work well; a higher neck on a less stretchy t-shirt does not.)
I don't find them particularly hot, but that's because I made my own and just attach my usual holster to it. It's easy if you have any craft or sewing skills -- I just took a worn out leather belt I had that looked terrible, lined it on the inside with a doubled-over layer of linen that was twice as thick as it was, and stuck velcro on the ends of the linen. The belt buckle sits on top of the overlapping linen inner band at around eleven o'clock, and the holster attaches to the belt at twelve noon.
If you're a woman or man who carries extra pounds around your waist (as opposed to hips), this can be a good way to carry.