The Martelli really does make a difference on strain to my arm, wrist & shoulder compared to my Olfa cutters. It's easier to cut curves with it, also since its easier to control. I also agree with the previous posters that Martelli blades make it work more smoothly than others. My only complaint with mine is the fuzz that works up in it and has to be cleaned out. My Olfa doesn't suck fuzz up inside like the Martelli one does.
Oh, I should also point out since the Martelli is smoother to roll, it's also easier to nick fingers that are hanging over the edge of the ruler!