I have not covered Curves, although the design does look like it will tone and ease the strain on the joints, but I have no first hand experience of this model, and I don't know anyone who has tried the shoes.
We have exchanged a couple of emails since you asked the question, however I wanted to add what I have written so others can benefit too.
You mentioned you have bad knees - years of high impact sports take their toll, and toning shoes are a great way to lessen the strain, and to start with, I think the following are a great first choice.
Original Skechers Shape Ups / Skechers Tone Ups sandals
Avia Avi-motion range (iburn, ishape, iquest)
The above are all well priced and highly cushioned and should make walking less painful. MBT Shoes and Chung Shi would be my first choice, however they are expensive and whilst the extra money spent will provide good value in the long run - they are designed for a long lifespan - all of the above offer a cheaper way to see if the shoes work for you.
Let me know how you get on with whatever shoes you decide to buy!