How effective is TaeKwonDo when it comes to street fighting?

I just got my green belt the other day, and I'm really enjoying TaeKwonDo, but I'm curious as to how useful it would be if someone actually attacked me. We've learned self defense things, but I'm not sure. From your experience, how useful is it? Do you have suggestions on what I could take along with TaeKwonDo?

my friend has been doing karate for about 15 years now and earned national championship titles but hes also done alot of training in other styles. he's has really good technique and in play fighting hes showed us some pretty incredible moves hes learned. but he doesn't actually get into fights, he just trains to the point of excess. last week i watched him get beaten down by some crazy drug addict, he was trying to use his MA moves but they just weren't working (i don't intervene in one on one fights). after getting bashed for about a minute he gave up and apologized to the junkie for giving him lip. i think its obvious why he lost. despite all his training he had no actual experience on the street, where as this homeless meth addict would have been a veteran. so my question is, could a martial artist with, say 25+ years experience who never actually fight beat a streetfighter? i used to believe it but after what i saw last week it makes me seriously question the effectiveness of MAs.

what is the best martial art for street fighting? don't go give me krav maga TKD or something stupid!! Good answers!