I love love love my Martelli cutter! I can cut all day, very accurately, with no fatigue or stiffness. I've tried pretty much every rotary cutter available, and the Martelli is definitely my favourite. The only thing I don't use it for is paper piecing, where I prefer the Kai cutters that automatically retract the blade.
I also really like their blades. At half price, they're a great deal.