I love it. On just about any girl. However the piercing and the jewelry have to match the girls style. Cute, pop style girls look good with the small (often jeweled) studs in their nostrils. More edgy girls, perhaps punk or rocker look good with a hoop in their nostril or a septum piercing. Goth girls and nostril w/hoop go together very well. If she has the right look and attitude, a girl with a septum ring, that's super hot in my opinion. I love body piercings. I have my nose pierced with a small stud, it has a black top that matches my 2ga plugs in my ears. I also have my navel pierced with a small hoop. :p I think I went a little overboard with this answer.
Stud nose piercings I don't mind, I'm pretty indiffernt to them so long as they're not huge and she can pull it off. Any other kind of piercing (aside from the ears) is a complete turn-off for me, I don't know why but I just don't like them at all.
And if she has a tongue pierciing, I won't even try to deal with, she'll have to choose between it and me.
I believe that if you have the right type of looking nose, then they can and do look very awesome. [&sexy] I don't think they look trashy at all. Its the one facial piercing a girl can pull off with out making it seem like she just walked out a trailer park. go for it!