I have a Martelli mat, and I love it. It's much better than my previous Olfa mat - totally worth the investment.
The Martelli mat is much thicker than Olfa, Omnigrid, Fiskars, or other brands I have found in stores. They claim it doesn't warp even if you leave it in the car... I haven't tested that, but I believe it because of the weight.
My mat is yellow on one side, and mauve on the other. This provides nice contrast with fabric, making it easier to line up cuts. I use the yellow side most of the time, except when I'm cutting yellow fabric.
It also really is self-healing; the material it's made of is does reattach itself across cuts. They show this in one of their videos, but I didn't believe it until I saw it myself. I also find that my rotary cutter blades last longer now.
All in all, my Martelli mat is the best mat I've ever encountered. My one complaint about it is that I don't think the lines are as accurate as they claim. I just laid my 24" Olfa ruler on it. One end was aligned perfectly with the mat markings, and the other end was off by a small amount. 1/64", maybe? You can see it in the picture. It's not enough to cause problems If I sometimes measure with the mat and sometimes the ruler, except when I'm cutting really long borders, but it does bother me.
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