I'm a big fan of these shoes. They're comfortable, and I can definitely feel the extra muscle workout (esp if I haven't worn the shoes in a while). I'm going to keep my review pretty brief, but address some common topics.

-With both pairs of Shape Ups that I own (the black and white sneakers and a pair of boots), my true size fits me very well. Based on my experience, I would not recommend sizing up/down.

-These last forever, at least if you're lighter than average. I remember (somewhere) reading  review saying that the sneakers fall apart quickly. I've had mine for two years and, while I don't wear them consistently, I've put many, many miles on them. I was kind of hoping they'd wear out so I had an excuse to buy a pair with a color scheme I like more, but no luck! My friend pointed out that shoes wear out faster the heavier you are, so maybe my shoes aren't wearing out because I'm very short and, therefore, lighter than the average woman. (My weight fluctuates between 100 and 110 pounds, if this helps.)

-I read several comments about knee pain from wearing these shoes. I've walked 8 miles in these shoes in the past two days and my knees have been aching a bit. However, I also often wear platforms, heels, and have had issues with my knees popping for ages. So I don't know whether my latest experience of knee discomfort is related to the Shape Ups or not, but I figured I'd mention it.

-My only other experience with toning shoes are a pair like Reebok's Easy Tone and Earth (negative heel) shoes.  As far as feeling my muscles working, I definitely prefer Shape Ups. I don't think I'll be buying more Easy Tone style shoes in the future, but I do love Earth shoes!