Here's my original post about this: back in September 2013 (but the event itself happened before that):
It wasn't really a no-strings sex website, but I replied to an ad about 4 years ago that was in the "casual encounters" section of Craiglist (which doesn't exist any more). Whatever I wrote must have worked, because I got a reply, we exchanged a few emails, and arranged to meet that Saturday evening. I turned up, expecting either a no-show, or a complete minger to appear. The girl that appeared was genuine, Indian, petite, sexy, intelligent (a doctor). We had some drinks, some dinner, we snogged, I took her back to my place, we shagged a couple of times (for the record: 6 out of 10), and she spent the night . She left the next morning to go to work. All good so far.... she then proceeded to bombard me with text messages and emails wondering why we weren't meeting up again, how she thought we got on really well, how she couldn't believe I was just after one thing, blah blah.... I did point out to her that that was what the "casual encounters" section of Craigslist was generally meant for, but she was having none of it. Eventually I had to change my mobile number and disable the email address I'd been using for such things, so she couldn't get hold of me. I don't live in London anymore, but for a couple of years I was always slightly on the lookout in case I ran into her.
So anyway, in short: I am living proof that these websites - Craigslist at least - CAN be genuine, maybe very occasionally. But if there are any genuine girls on there, they are probably total nutters. So the conclusion still ends up being: avoid!