The only thing I've found dynamat to be useful for is keeping the car quiet from the outside a.k.a. neighbors can't hear your car's sound from the outside.
Other than that, I think it's a waste of money. They claim better SPL, but the only time that dynamat increases SPL is when the vehicle's body panels are wobbling like a piece of paper in the wind. You won't see that kind of flex on most cars.
On any car where you have that much flex in a body panel you are wasting your money to buy dynamat. In a case like that, it isn't the elastomeric properties of dynamat that are stopping the vibration, it is the sheer mass of the sheet and so you might as well get something cheaper like FatMat.
If you drive a crappy car like the Saturn SL2 that Dynamic Control used in their 3db gain test.... a car that rattles when you sneeze too hard, then yes, you may see an increase in SPL. But again, it is the sheer weight of the dynamat that is main reason the panel stops vibrating, not the fancy elastomeric properties.
If your body panels flex like cooked lasagna noodles, then you are are much better off either reinforcing the panels with fiberglass, or using a cheap dampening pad.
keeping outside noise out/inside sound in will make a big difference. also putting it behind your speakers and on the panel they are mounted on makes a huge difference. but it all depends on the equiptment you are running i say.
if you have that much body flex, then you need to be looking at sand and concrete, lol.
i agree its not worth spending tons of money on and thats why i said theres cheaper stuff out there. if i can find it, someone did a review on a couple of the main brands of sound deadner out there. i'll post it if i find it.
nothing can help i saturn imo with its all plastic body panels, lol.
just dont expect a miracle or anything with that stuff.