I use mine all the time when doing paper/foundation piecing. I can't imagine how one could do that without one. I have a 6 inch and a 12 inch, and they are both worth every single cent I paid for them.
Yes, they are worth it if you are going to do paper foundation piecing. I couldn't do it without the ruler. I have even used the 1/8" once when I did a miniature foundation quilt.