I have a corset that I wear for ren-festivals that's extremely comfortable. I tend to slouch, but the boning in the corset makes me stand/sit up straight, which obviously improves my posture, and causes me fewer back problems. I'm not accustomed to wearing it, so I'm usually happy to take it off at the end of the day, primarily because my back muscles aren't accustomed to having to do their job as intended. It may not sound logical, but after wearing the corset, instead of hurting, my back is just tired. I also have a girdle-like contraption, but I've not had the positive experiences with that. I've found that it tends to roll down at the top. (What's the point of having something suck in your tummy and midriff if it's just going to roll up around your belly button?)
Anyhow, I'm not suggesting that anyone rush out and buy either, I just thought I'd mention my positive and negative experiences with both. (Un/fortunately for me, they've both "shrunk in the wash" so I can't wear either of them at the moment.) Sarah