I have bought the foot but never tried it. Yesterday I stopped in at a Quilt Store that also sells Berninas and the owner demoed the foot for me. She was using a nylon cord called La Espiga. It is used in macrame I believe. I was very impressed with the beautiful design and how easy it was to do. Having said that, it always looks easy to me until I try it.
For example, I tried to load a practice piece on my quilt frame that I had for over two years. It looked easy in the pictures. Well, I must confess to very unladylike language and no success.
So, who knows what I will do with the couching foot. But if you're into embellishments, I think it's probably worth it.