I once had a date with someone I met online, not through a real datingservice site though. It was ok, I made sure to meet him in a place where lots of people were and we had a nice meal. Nothing terrible happened and he was not a creepy killer. I consider him a friend now because we were not real compatible romantic wise.
I know some couples who did meet with the help of a online datingservice and they are very happy. I wouldn't try it myself right now because I'm not looking to meet someone at the moment but if I was I would consider it. Internet is a great way to meet people as long as you always remain careful and don't hook up with every idiot you meet online.