If you decide to do the entire floor and the doors, yes, it will cut down on road noise coming in.
It's dual purpose. Some people will do the entire car, include taking the headliner down. It adds weight though, so you have to be sure you want to go through all that effort.
You guys i got it at crutchfield and its at best buy(not alot tho) Best bet is on amazon, 25% cheaper too. It is quite expensive, ive sunk 200 bucks on just what you see there. It quiets noise anywhere you want it gone. They use it all the time when restoring old cars.
Also there is DYNALITE, its thinner and has a blue backing instead of silver. Dynaliner is great on the roof and trunk too(foamy like, and its applied on top of dynamat). Working my way there.
Dynamat is great. Sticks great(to anything), looks great, and you get to say you have "DYNAMAT" lol. I have not used any other sound deadening products. I got it because of its good name and I have seen the befores and afters of it. I buy bits here and there. Shop amazon for the bulk packs. Other than that, you get what you pay for.
Update on the PRICES and I also ordered some more dynamat. Roof will be finished and the bottom side of the 6x9 speakers by the end of this coming week. Should make a big difference! will post more pictures.