Cor-ten steel is a beautiful material. It is being used in gardens more and more and loved for its warm, weathered and industrial look. In my experience, this edging looks better when it is 1/4" thickness. Anything thinner and you will see bowing and the edge will not appear clean and straight, which is it's best attribute.
Also when specifying corten, make sure you get a certificate that verifies it is truly cor-ten, and ASTM grade A588 or A606.