I also have the Martelli double-sided mat and rotary cutters. IMHO if you can afford it, they are worth the money. The mat has accurate measurements on each side (light side - yellow and dark side - lavender) and it is not supposed to warp if left in the car. They say you can even iron on it if you follow instructions. Here's the website:
http://www.martellicatalog.com/mm5/merchant.mvc? I saw them demonstrated at a local quilt show. They also have video demonstrations online. I called and ordered mine over the phone. I got a better deal as well as fast delivery.