Adding sound deadening material. Downsides?

[FireFox31]

I also put Noico sound deadening material in all four doors. Whenever it’s time for a new roof liner I’ll put Noico in the roof too.

I'd be grateful to hear anyone's experience with adding sound deadening material to their car. Where did you put it, what brand did you use, how much of an impact did it have? Are there any downsides to adding this such as heat buildup from less airflow?

I would put this material in the doors, tailgate, and roof. Since I'd have other trim out to remove the headliner, I could put it below the trunk windows and on other side metal (pillars).

Still, the sunroof whistle might make all this soundproofing entirely irrelevant.

[kallekula]

Go nuts. I used Noico as it says above. Some people say it’s cheap and therefore not as good(!?) Other brands are significantly more expensive. Whatever xtra deadening is achieved wasn’t worth the money at the time i did it.
So far I’ve done all doors and I plan to do the roof also when I have more energy.
Floor isn’t worth it. The carpet in my bmw has like 4-5 inches of padding. Btw all this was done to my bimmer. My s70 would require too much work to quiet down I suspect .