I find sewbizgirl's comments interesting because I've used Olfa mats -- ALWAYS purchased from Hancocks of Paducah, never Joannes, etc. -- for well over 30 years, wash them occasionally as Olfa advises, and have NEVER had a problem with them not 'healing'. I do use both sides of my mats, and change up where I cut on them so as not to cut over and over again in the same groove, but I've used these mats for years and years without causing a 'bad spot'.
On the other hand, in my experience in many years of classroom situations, I've decided the Fiskar mats may be the worst ones on the market for dulling blades! I also use only Olfa blades, by the way, which I buy at Hancocks of Paducah in the 10-pack for a good price.
Jan in VA