I like to listen to loud music sometimes. I've blown out quite a few speakers and I have actually had the "can't hear emergency vehicles" experience - I wondered why everyone was pulling over until I looked in my rear-view mirror.
I was driving a 1966 Dodge so it wasn't a soundproofing thing.
However, it embarasses me when others can hear it because I'm afraid they'll think I'm doing it to impress, which is just kind of tacky and immature, in my opinion. Plus I just don't want other people to know what I'm listening to, that's my business. It's kind of like having a bunch of people overhear your public cellphone conversation - even if you don't care about being inconsiderate, now everybody knows your private business. Basic respect is a huge factor too, since not everyone shares my taste in music. So I turn it down a little at stop lights or when driving through residential neighborhoods. I usually have the window open but it's for fresh air (no ac). There's a lot of music I can't stand being played really loud (rap, mostly) so I try to show the same consideration I wish they would. God knows some of the stuff I listen to could easily give children nightmares.
I didn't quite know how to vote on the poll because I think loud car stereos can be "cool" in the sense of "cool, my music sounds really great when I turn it up and I enjoy listening to it" but not "it's cool to blare your music loud enough to wake the dead."