I have and am currently on two dating sites - PlentyOfFish which is free, and Match which is a pay site.
The numbers on POF are good, but the quality is very, very low:
* Fake profiles (easy to spot though)
* Unemployed single mums with multiple kids who live in a craphole of an area
* Cougars (Think Dorien from Birds of a Feather) looking for toyboys *shudder*
* Lots of lonely girls who just want to "chat" but have no intention of going on a physical date
Match is a little better but:
* Very expensive for what it is
* You have no idea if the girl you send a message to can actually read your message as she may not be a full member
* Some girls have a list of requirements longer than you arm
* If you live in a non-metropolis area, your options are restricted
Saying that, it sounds like I've had a bad time, but in the last year I've probably had about 12 dates, of which 10 were great and 2 were disasters (old pic and a cat lady). It really is a numbers game though. The only way that you'll get replies as a man is to send as many good quality messages as possible.