I lucked into a Shape-Ups style, Dr. Scholl's brand shoe at DSW (LOVE that store) this winter on clearance, plus I had a coupon...only $15, so I bought them to try them out.
At first, I realy could feel it in my calves. By forcing you to stand forward and more on the balls of your feet, it was like doing heel-raises. Even after wearing them just a bit, I really felt like I was getting a little extra workout. I couldn't "feel" anything in my butt though, so I didn't get that out of it like they advertise in the commercials. Now that I've worn them for three months or so, I don't feel that initial mini-burn anymore.
No back or knee pain, but something weird -- on longer walks, I began getting a little headache. It happened several times and only when I wore these shoes, so I began to wonder if they were the cause. It doesn't happen now that I'm used to them, but I wonder if...because the heel is "missing"...I was pounding the ground too hard and jarring myself. I started to pay attention to how I was walking and adjusted my stride...again, not sitting back on my heels, walking more forward and more smoothly. The shoes haven't given me a headache since.
Overall, I don't know if they're doing any good at shaping-me-up. I've always had "cheerleader calves" anyway. But they are really super-comfortable, and I do think my posture is better, standing taller, etc.