67stang7,
The stuff you're refering to may also be called Peel 'n' Seal. Those products are asphalt based building materials usually with a foil facing and a constuction grade adhesive. Up until a few years ago the products for cars were not much better than using these products. In fact, some companies repackage this stuff as their entry-level deadening product.
What these products and many of the newly developed ones do is called 'mass loading.' Essentially, by applying these products to body panels you are changing the mass of the panel and therefore changing it's ability to tranfer acoustic energy through the panel. Vibrations, at certain frequencies may be deadened and result in a quieter ride. Using deadening products in addition to absorbing products will yield the best results.
I personally would recommend using products designed for the automotive industry. The adhesives used are far better. PnS is infamous for falling off car roofs and door panels. Secondly, several manufacturers have developed better, more efficient compounds. The new products are not asphalt based which can smell when it heats up. Dynamat is the most well-known and most expensive. IMO you're payng for their name. Try looking at companies like Second Skin Audio, B-Quiet, or eDead. They all make decent products at much more affordable prices.
