I covered the inside of my Mustang up to and including the doors with Dynamat Extreme. It did help to drown out some of the bass tones from the mufflers and road noise but now I can hear the highs of the squeaks. I'll be going back to add some baffling when I take the dash off to do the firewall and front floorboards.

The most amazing difference was the doors which were covered both inside and on the exterior before the door skins went back on. The difference is noticeable but YMMV. It is still a loud car but that comes with the territory. I don't have to yell to have the passenger hear me now but it's still not good enough for listening to music. As a side benefit, the doors now close like their filled with concrete.

I'm not sure that the extra cost is warranted for the Dynamat Extreme. I would go with the cheaper home depot stuff if I was putting it in a daily driver. Just make sure it doesn't have asphalt in it. And get some seriously heavy gloves, several good utility/carpet knives, a 1" wall paper roller, and a heat gun. You will be amazed at how much your thumbs will hurt afterwards.