The only time i've ever called 911 (it's actually 911 here in Canada =P) was when I was really young, I can't remember what age exactly, because I was really mad at my parents because they had punished me for something, so I went up to the attic, which was my kind of hangout area, and dialed 911 and said my parents were being mean to me, then got freaked and hung up. They called back and I answered again, hung up on them, and then I think the police got ahold of my mom or something and I think I got a stern lecture after either denying the whole thing or claiming it was an accident. It was pretty scary at the time, but I find it funny now.
i've been around when it's been called before, but never actually needed to use it myself.