An uncle of mine met his wife of a few years on one and is happy as can be last I knew. Other people I know have met some really odd (and I mean kind of scary odd) people. I think it's more or less hit and miss just like in a bar, just on a wider scale. You do have the opportunity to meet people you would otherwise not have known existed because they don't go to the same social gatherings as you. However, the same rules still apply. Meet them at a well known, well lit, public place. Bring a cell phone. Let a good friend or family member know where you will be and about what time you're supposed to be home. Just because they seem nice doesn't mean you should be stupid about it.

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"BOY: On a hot summer night, would you offer your throat to the wolf with the red roses? GIRL: Will he offer me his mouth? BOY: Yes. GIRL: Will he offer me his teeth? BOY: Yes. GIRL: Will he offer me his jaws? BOY: Yes. GIRL: Will he offer me his hunger? BOY: Yes. GIRL: Again, will he offer me his hunger? BOY: Yes! GIRL: Yes. BOY: On a hot summer night, would you offer your throat to the wolf with the red roses? GIRL: Yes. BOY: I bet you say that to all the boys!" -Meatloaf