I fitted sound deadening foam rubber sheet under my carpet as I didn't want any resonance or booming inside the cockpit. Sure you can hear the exhaust but other than that my car is very quiet inside and often people comment on that.
It cuts down on the engine noise and gives the car a solid production kind of feel and it also makes the carpet squashy when you put your hand down to get in and out or knock your knee against the tunnel.
I chose foam rubber rather than the traditional felt in case the car got soaked in a downpour.
I used a heavier sound proofing material in the boot space.
I bought the stuff from this company...
http://www.noisestopsystems.co.uk/
I would definitely use it again.
...peter