Someone conducted a "Sound Deadener Showdown" and posted the results here.
Fat Mat is included in the list of deadeners tested. The article is lengthy and hard to find specific deadeners so here are some excerts about FatMat. Go further into the article to see the different tests done to determine how good each deadener truely is.
Peel & Seal and Jiffy Seal are roofing products that fit in this category. Many people buy them for sound deadening. Of the products I tested, B-Quiet Extreme, Brown Bread, Dynamat Original, eDead v1, eDead v1SE and FatMat are in this group. Many people report great success with one or more of these mats. They are certainly the cheapest option. If you know your car will never be exposed to extreme heat or cold, they may work for you. I'm not suggesting that all of these products have a 100% failure rate under extreme conditions, but I do believe they are risky. Their narrower temperature range also means they will be less effective the farther they get from room temperature. I would not use any of these products in my car.