I bought a set of them from Nancy's Notions catalog because I, too, thought they would sharpen my blades. But the 6 1/2 x 18 1/2 seemed to dull the blades, not sharpen them. However, I do like other features of the rulers: the non slip under surface, the easy to read measurement markings, & I also like the color of the rulers, even though they are frosted. I think maybe being able to see through them depends a lot on the color of your mat underneath. I have green mats & a purple mat. Actually, I'm going to keep mine & use the opposite edge instead of the one that's supposed to sharpen your blade.
If the manufacturer would forget that feature, I think this line of rulers would be a winner.
JMHO