Three times. Two in the modern, conventional, Craigslist sense and they were just okay.
The first, though, was in 1994! The (public) internet had barely been invented, and while I was in college some chick randomly got my e-mail address and started writing me about my school. I thought she was some enterprising young HS student who was scoping out colleges, but she turned out to be a college student as well. We struck up a three or four month correspondence (telnet anyone?) during the school year, and I made the booty trek up to Colorado that summer. It, too, was just okay.
I’m guessing you’re a guy. If you want to do it, post exactly what you want and make it sound interesting and exciting. Don’t expect too much in the way of some hottie knocking down your door.
If you’re responding to ads, expect a lot of spam posts that are for paid chat or whatever. Again, make your response interesting and exciting, and be patient. Also, be flexible with where and when you first meet, etc, which should be someplace public.