I'm very picky when it comes to choosing things like that, and my criteria would probably have a few more bullet points than most people. All in all, the site has to be reputable and with lots of customer reviews and testimonials.
I've never bothered to check on the dating sites before but if I was to do this, the level of maturity and dignity in these sites would be vital. Dating isn't so easy after and for starters, you have to get it right from the beginning.
I'm wary of all kinds of dating sites out there, and I would have to have tried and failed at everything else before considering this as my course of action.