You won't notice a huge difference, but it definitely helps. Anything you can do to help deaden the car is good. All that rattling and stuff vibrating is just a waste of energy, that could otherwise be used to create sound.
I personally think regular Dynamat is a rip-off, the product is good, but there's plenty of other brands that work just as well for a lot less. (Sounds like JL, hehe)
There's a few differen't liquid sound deadeners - they work good for tight areas and sharp corners/edges that it would be really hard to put a mat on. The liquid stuff looks better since it's one big perfect coat.
The roofing stuff from honedepot works pretty decent. I think it may have tar in it though, so on hot days it stinks somewhat.
dynamat definatley helps it made my subs sound better on the higher fq b/c my trunk would flex and all i could hear was the metal rattling. it makes the subs have a clearer sound also depending on much dynamat you use you will notice a big increase in spl. on the Dynamat packaging it says if you use the full extreme kit in your trunk you could have as much as a 3 db gain. if you go w/ dynamat get the extreme the original sucks doesn't stick to wear you put it and not as easy to work w/ as the extreme.
joe i kow you have to heat it up. since i live in florida i used to set out the dynamat outside for about 10-20 min( during the summer when it's almost 100 out!) and then i would go to work but even doing that the orignal just obviously isn't as good as the extreme.
mike yes and no, if your subs don't rattle a whole lot to begin w/ and you sound deaden the entire vehicle you may not be able to hear bass outside the vehicle at all. w/ my car it still rattles a little but you mainly hear the bass and not the rattling of the trunk.