For me, it depends on the person. I don't really mind it if it looks good on the person. I'm not big on appearances unless it's a celeb or fictional character or something.
However, my dad has a mustache. He's had his mustache since before I was born. If he were to suddenly shave it off for any reason, I would be devastated because I grew up seeing him with a mustache my whole life. That very same mustache he has never once THOUGHT of shaving off. Though, he has joked about it.
Yes, I'm aware this thread is geared towards guys, but hey... girls are allowed to give their opinions on facial hair too, right? :P