Mrstampe,
While not useless, its difficult to say if the specific type of egg crate you pick up is indeed able to provide a good deal of acoustic damping. If you can blow through it then it will have at least some benefit. Through all my research I found if possible get the rigid, aka 703 or 705, fiberglass and if not use the fluffy pink stuff. Both work well and while the rigid is the best it is difficult to find and usually pretty expensive. Here is a great site that compares a ton of stuff:
http://www.bobgolds.com/AbsorptionCoefficients.htm http://www.foambymail.com/Eggcrate.html (has values for the egg crate)
In all, the fluffy insulation found at the local hardware stores if the best option, providing more acoustic damping than most acoustic foams and is very cheap. The only downside to this application is the health hazards.
One question that I can't find an answer to is if the R-value of insullation has a direct realationship to its acoustic damping properties. For example, OC makes 3 types of 3.5'' insullation with R-values of 11, 13, and 15. Since they are all the same thickness I'm not sure if they all perform the same or if the higher R-value indeed helps.
Hopefully this more than answered your question.
Nate