My car, an older Toyota Avalon, was solid and pretty quiet to begin with. I bought the "trunk kit" for installation in all 4 doors and the driver's foot well and think it helped, but that may just my mind trying to justify the expense. This installation was done as part of a component speaker upgrade for all four doors. The difference in sound quality is phenomenal, but I think most of the credit there should go to the new speakers. Installing this stuff is... there's no easy way around it... time consuming. I spent 2-3 hours PER DOOR (in addition to speaker installation time) for 70-80% coverage. For resonance damping, Dynamat's instructions indicate that at least 30% coverage is recommended, so my job was overkill in that regard. My goal was also to reduce outside noise intrusion into the cabin, and I think I helped there. Most of the remaining outside noise is coming in through the greenhouse instead of door and floor now. I recommend some oversized pieces of paper (flip chart paper, kraft paper, or, in a pinch, newspaper) to serve as templates -- mark and cut those prior to chopping up your Dynamat. It's such a no-brainer, and such an expensive box of product that I'm disappointed that Dynamat doesn't put some template sheets inside each box. The product can be cut with sharp scissors or a utility knife, and the remaining edges are sharp enough to cut YOU. Use caution, perhaps gloves, and a roller of some sort to keep from leaving blood behind.
Pros: Fairly lightweight and thin, easy to work with and to mold into irregular body panels. Does a good job of killing body panel vibration, and a mediocre job of sound insulation.
Cons: Expensive. This package was more costly on a per-square-inch basis than some of the smaller Dynamat kits.