The only times I regularly took cold showers was at home during the several days we had no hot water due to hurricane caused power outages and boy, I don't miss them and would never purposefully take them.
While I found my body did get used to it after a minute or two exposure, but that first icy blast was brutal and had me gasping for breath. Since we had no A/C either and it felt like the very bowels of hell inside during the day; it made cold showers a double edged sword. While I admit the cold water initially felt like paradise from the stifling heat, but when it was over I found I went back almost instantly to being a sticky, sweating mess even worse than before I got clean. So I imagine they might feel good afterwards if the room is cool and climate controlled, but if it's 90 degrees or more, forget it.
But as far as increasing testosterone, not sure who would claim that since I thought they were supposed to be a libido killer hence the old advice of "You need to take a cold shower" when someone is overly horny. I certainly know they do a good job of causing maximum shrinkage of the dangly parts, which having to watch them almost disappear back into my body would put me in less of a sexy time mood.
