They CAN look nasty, but it depends on the individual. Someone who just tosses their comb out will probably have a dirtier looking head of locks than someone who pays hundreds of dollars for a salon to do theirs.... and some people do. There are varying amounts of effort one can put into the look, from doing absolutely nothing to hours of work.
I love them, when done correctly (and I don't believe the salon is the inherent correct way; I believe careful consideration of how they fit YOU is the correct way). I have locks, my brother has locks, my uncle used to, my boyfriend does, my aunt does and so does one of my cousins. They don't look right on everyone and not everyone chooses the correct technique or size, etc.
When people say that people with locks "don't want to be taken seriously," it makes me laugh. I think one of the biggest and most common things that it says is that we don't feel the predominant standard of beauty is the only standard. I don't put poisonous chemical straighteners in my hair to emulate a hair type that I will never have, but there will always be someone who says that a black woman who wears any type of natural hairstyle looks like she doesn't want to be taken seriously. Oh, well.
In my personal opinion, many white people's dreads don't look that great, but I think part of that is because of the wax and other crap that they use. That stuff is gross and leaves a residue that will basically never wash out and will attract dust and lint like nothing else. That hair texture also tends to simply clump together rather than curl and twist, so it doesn't look right to me because of how I prefer locks to look. That being said, I've also seen some that look pretty good, and to answer the general question, no they are not a turnoff... many people see it as a turn-on. I quite possibly never would have spoken to my boyfriend if he didn't have them. They were what caught my eye first.