Hi Barb! I am one of those whose ruler seems to slip frequently. I have a problem with my arms and have less strength in my hands than I would like (have metal in both arms from shoulder to elbow). My husband is going to get me one of the ones at Harbor Freight near our house so I can try it. My quilting instructor tells us all the time not to use the grid on our mats because they can be inaccurate and a variation of 1/8" or less can be a lot more by the time you get to the end of a quilt.
Linda