I've used them once, and am thinking about getting a pair. I enjoy running on land, but my left knee sometimes bothers me. I'm thinking that for those times, I could run in the water. Competitive runners use deep water running as a way to stay in good running shape when they have an injury. The shoes simulate running more than just running in deep water, so it adds more resistance and works more muscles, burns more calories, etc.
Before I spend the money (although they are less than regular running shoes), I thought I'd see if anyone else has ever used them and what the verdict was.