I find it ironic that Matt used a product that didnt have "mat" in it...
I have the FatMat and can say this from my experience. I think it does an outstanding job of keeping the stereo sound inside the rig. When I turn it up and step out- its very muffled to the outside world. This is good to me as I dont want to be one of those types who blast out others at intersections.
Unless you have some way of filling every nook and cranny, I cant see this helping much with road noise. The windows on the FJ and doors are thin and even with a great coating, dont make it sound like a Mercedes Benz. Unless you added like 400 lbs of the stuff, I doubt it will impress you that much.
What it does do well is make the small nuances and subtle sounds in certain music listenable while driving. Its like a "loudness" control for those sounds. Mainly in Jass and Classical which most of our members do not subscribe to. For the hard rock, dance music - it boosts the bass. This will be noticeable in the fact you dont need to turn it up as much so it does dampen some sounds outside but I believe it actually helps to keep interior sounds inside the FJ, making it seem louder.
I found a spray on type sound deadener and thought that may have been a better idea- covered 20 sg/ft for $10 a can. That used in conjunction with the rolled on mat would be better.
The main issue I have with Fat Mat is that it leaks. Ive found many spots where the oil based material inside has become liquid and seeped out. Its for this reason, I didnt use it for the headliner, fearing it would drip down and stain.