The sound deadening stops vibrations more than noise. They add weight to stop the panels resonating.
Definitely worth doing on the doors , as well as sealing the service holes. It makes a big difference to the speakers as well as blocking some of the unwanted noise.
To get rid of road noise, engine noise etc U use sound barrier underlay. Its usually like compressed shredded felt/cloth with a tar layer in between.
I use this stuff :
http://www.knoxautocarpets.com.au/sound_proofing.htm
The sound proofing comes in rolls of 1 meter by 1.5 at $45 per roll.
Also make sure you seal the service holes in the boot , and make sure there are no holes in the firewall.