Thanks.
My Olfa mats seem to just go on forever. I've never replaced one, and they look pretty much like new. I use the tip picked up here and use one of those net bath scrubbers to give my mat a good going over every so often and that gets the fibres stuck in grooves. With the fibres gone, the mat closes up again.
I imagine a harder mat would dull blades faster. You've confirmed my sticking with Olfa!