I think it can be hit or miss. I remember reading somewhere here on the board that you should set up an ad yourself and then screen the replies. I set up one just to see what kind of response I'd get and well I didn't respond to any of the emails I got.
I did respond to one guys ad and we went on a date after talking online for a few weeks. He's very nice. A little short for my liking but he's a nice guy. We're just friends. I met an ex online a year or so ago. It was really weird how we met though. Almost freaky. We talked for a few months before we met in person. He's a good friend now too because he had some other issues he needed to work on.
I also have a few friends that I became tight with via Myspace/Facebook. We had mutual friends in common and ended up meeting that way. I've only had positive experiences. Even some of my friends became friends with those friends because we would all interact so much online and in the same college circles here in the area. It was like everybody knew somebody in common.
I do have to admit I was very skeptical at first. IDK if I'd do the Craigs List thing again. It just seems real impersonal to me. But I guess it's really no different than the way people used to put Personals ads in newspapers. Even when you meet people out in public in person you're taking a gamble. But with that at least you get to see them first.
ETA: After reading that article Always Wear Joy posted I might check into that strictly platonic section. I never thought about looking in that section.