One other thought... Not all blades will fit in all cutters. You need a Fiskars blade for a Fiskars cutter, an Olfa blade for an Olfa cutter, etc. The center holes can be different -- some with slots, some without.
An interesting observation we've come across at the quilt shop, is that if you have a granite counter top under your mat, your blade will wear out really fast. It doesn't make any sense, but we've seen it a few times.