I just bought 100sq ft of Fatmat sound dampner off ebay pretty cheap. I was researching, and its pretty much the same exact thing as dynamat except without the high price tag. I know people with Dynamat, so i could compare. I used probably about 45sq feet on the trunk alone. I did 2 full layers throughout the entire trunk, one layer on the celing, and 2 layers on the wheel well areas. Didnt do the cabin yet.
I can tell you this though, i had my interior BARE before doing this, only passenger and drivers seat installed. No rear panels, no carpet, no rear seats...nothing at all. Not even door panels. Once i got the trunk and the wheel wells done and went for a ride, it was about 90% as quiet as it was when it was fully asembled. I also then just took that egg carton foam stuff you use like under your bed and put that in the wheel well area behind the big black panels. Now i still have no rear seats, no carpet at all and its a bit quieter than it was fully assembled. I was pretty suprised, and im not even done yet i still need to put it the rest on the roof and the cabin.

Its not really needed on the roof, but i thought it was pretty cool instead when it rains of getting that hollow "ping" sound it kinda thuds instead. When you bang on the wheel well where its fatmatted now it gives a solid thud instead of that high pitch ping as well.

Im happy, only thing is make sure no moisture or water gets under it or you will have a nightmare like i did pulling it all up to find the leak