I actually what you might call a horror story experience with online dating, though I blame it on my own naivete more than the medium. Had a girl actually message me and say I was cute, which was a new experience for me. That being said, talking to her even via messages on OK Cupid quickly raised doubts about our compatibility, but there was one thing that suckered me in; she had problems, and it's damn hard for me to pass up the chance to try and solve other people's problems. Which of course often means I bite off more than I can chew, and good god did I bite off more than I could chew here. Her mental state was far, far worse than her initial messages let on, and her initial messages already didn't paint an image of mental stability. I just ended up getting sucked in deeper and deeper trying to somehow help her out of the horrible position her severe mental illness put her in, but eventually (thanks to the advice of some friends) I had to bail out to keep my own mental state in tact. I honestly still feel bad for completely bailing on her the way I did, but I realize that she was beyond my abilities to help. I mean seriously, she was messed up to a degree I didn't even think actually existed, and she refused all attempts to get her professional help (she was terrified of being institutionalized) so I was kind of stuck.
For the most part though the way we met doesn't actually have a lot of impact on the story; this could have almost as easily have happened if we met in person. Except that I learned part way through my dealings with her that I wasn't the first guy she had contacted like that through the site; she was kind of using it to "prey on" naive guys like myself. Though "prey on" makes it sound more malignant than it was. So I guess the moral is just be careful what you get yourself into, since online profiles can hide a lot of warning signs you'd normally notice right away.
Honestly this experience didn't make me think badly of online dating despite what you'd think. It was my own doing, and honestly I saw at least some of what I was getting myself into. I just overestimated my ability to handle it. I see a lot of potential for it if you put more effort into it than I did (I think I used a webcam picture for my profile, so it's hardly a surprise she was the only one I got). It's a way to meet people when it'd be difficult otherwise, nothing wrong with that. Just have to take some extra precautions since you can't confirm some facts as easily over the internet as you can in person.