These are extremely comfortable.  I walk my 85-pound german shepherd every morning for about 45 mins, and was looking to energize that experience.  Well, these shoes did the job.  They make me stand up straight (somehow, no slouching at all!), and walk a lot faster and brisker than I normally would.  My doggie tends to go too fast sometimes and i have to correct him to stay next to me at my pace - with these shoes on, we tend to be on the same speed most of the time and very little, if any, correction is needed!  Walking in these are like automatic powerwalks, like walking at a brisk space on a treadmill - very different from the slower and sleepier walks we sometimes end up with in the mornings.  The difference is very obvious right away, and it feels great. I usually wear a EU39 and that fit me fine in this pair as well. I also have lower arches, and that's a non issue with these as well.

These do seem to work the butt a lot more than my other walking shoes - i can feel the difference.  But I suspect unless you have or plan to have a regular walking regime, just wearing these around once in a while for errands is not going to make a big difference in terms of muscle toning or apperance.  The TV ads for these are def way over the top, but i guess they do get your attention!  Going forward I plan to wear these for the doggie walks - lets hope they last.  I think if i wore these all the time instead of regular walking shoes, the design could throw off my gait and create bio-mechanical problems, so I would rather be cautious.  In terms of energizing my daily walking, these are fantastic so far.