I've never used one but I guess you have to be very careful who your talking to but otherwise it could be great. My cousin met a man from Spain and they were talking for months finally she flew out to see him and he moved back to Canada with her. Now their happily married with two children!
I consider certain sites a good opportunity to meet someone. I used harmony.com and it was very successful for me. Some sites are better than others at providing compatible matches. I was not interested in meeting and dating numerous women. I was hoping to find someone without a great deal of hassle. It worked for me.