It doesn't look that hard to do, just take out the interior, carpets etc, stick the stuff down and put it all back together. The question is "Is it worth it"?

What sound deadening material are you thinking of using?

Actually IMO the standard carpet underlays and foam do a pretty good job already, that is if you mean for an SW20 (I can't comment on the AW11 or Spyder). I've taken a fair bit out for lightness sake as all up it's quite a few kilos (I don't really drive mine daily so I can put up with it) and I can tell you it's quite a bit noisier. However since the cabin is quite small it doesn't have the same "tinny" sensation like if you were to strip a conventional car.
 
Just remember that putting an SW20 interior back together is a PITA and if you want to do it nicely I'd reccomend getting new interior clips since half will probably break off, otherwise the panels will never sit right and probably rattle (offsetting any sound deadening benefits )
 
Sorry, that wasn't really a clear answer, just my 2cents.

Good thing it works if installed properly.
Bad thing the weight of the material.

I have a box of dynamat in my car. Good stuff, keeps a lot of the road noise out and makes the sound system sound pretty good. Car feels cooler inside for longer too

I have used DYNAMAT (original) on a few of my rides..if installed correctly, there are definitely benefits:
The cabin stays a lot cooler, outside noise is a lot less (if all door seals are still good). I also used them inside the doors and the roof...the doors feel a more 'solid' especially when closing the door. Then, if you have a sound system in your SW20, you can enjoy the fine sounds in the music.
 
(I prefer to listen to the music coming from the 3SGTE behind me...without the heat on my back!)
 
P.S. If weight is a serious factor for you, then I say no Dynamat..  it will be heavy!
 
My 2 cents...

Has anyone used the Dynamat Superlight?
 
http://www.dynamat.com/automotive-and-transportation/automotive-restoration/superlite/
 
I wonder how effective it would be in comparison to the Xtreme and how much less weight it would add?

Yeah I forgot to mention that I have always used the 'Extreme'...so not sure about the effectiveness of the Superlight ...use the Extreme and just turn the boost up a little  to make up for the weight heheh!
 

There are a lot of misconceptions about how to actually achieve good results, leading to people carrying a lot of ballast for no reason.  Check out this site for some great insight,
 
http://www.sounddeadenershowdown.com/
 
I have used some of these techniques with decent results.  My issue is strangely enough this becomes an obsession.
 
Is it worth it?  I guess that depends on your reasons.  Mine were my daily commute on a HWY with an ordinary surface and audio quality.  You can gain some serious SQ through doing this the right way (giving you better returns on your $$ spend on equipment), not to mention making your cabin a more comfortable place to be.
 
Let us know how you get on.

+1 - I'm going to try this on my SW20,. Currently, I don't bother turning on music while driving.
The bare minimum would be to stuff the side qtr panel (or whatever you call it) behind the driver's seat. I get tons of I/C fan (9") and tyre noise through there.
 Driving on the M1 into Sydney from the north, the surface changes from concrete to bitumen and the reduction in noise at that instant is tremendous, which helped identify the problem/solution. 
 I intend to do many more panels than just that one, but for those who couldn't be bothered doing the whole car, I recommend that panel, probably done in an 30-60 mins.