I would guess it is very hard to find someone that is truly serious and follows through. That can be said for many aspects of life, online and offline. As far as success rate, we wouldn't know since they wouldn't be around any longer to report it. The only updates you are going to get are those from people who aren't successful or having bad experiences.
If you try, just be alert, take your time to try and get to know the person you will potential meet, don't let people try to talk you into things you are not comfortable with, don't let anyone rush you, and if at any point you are not comfortable with, or become suspicious of, the person you are talking to, say so and move on. If they harrass you, or you feel the person might be dangerous, let the mods know and they can look into it.