When i had my celica not too long ago now i had Cascade Trunk kit installed and i got dynamat for the front, it's not that bad esp when cascade product you can spray on but, i'd becareful about the spray on , sometimes you can go really thin , so in hindsight i'd just get the mats like dynamat makes or cascade audio makes and put it in. I had 2 15 inch subs and i didn't even hear a rattle from the outside of the car .
This stuff rocks, costs MUCH less than a full dynamatted car. Right now they have a sale on a 100sq ft roll which will cover most the car. For the same price of dynamat you'd get enough for 4 speakers.
Personally I have 100sqft in my car, everywhere but the footwells and headliner. I used the RAAM product offered back in 2001 (not available now). I did it for the overall improvement in sound quality not bass/rattles. Road noise is significantly reduced.