I met a couple of great short-term relationships on on-line dating sites, but strangely enough the only relationships that made it to long-term were with people I met in real life.
In regards to weeding through all the spammy messages, the sites you have to pay for tend to have less of that. The more it costs, the less people will be on there simply to waste your time.
The question system on OKCupid (you can answer as many or as few multiple choice questions as you like, its a little addicting like a game) is really great, because it allows you to weed out alot of your deal breakers before even talking to a person.
As a female, try browsing through the profiles and messaging a few guys you like, yourself. Guys don't tend to get alot of messages, so it'll probably make them happy and you're likely to get a response. Can be much more positive than wading through the 100s of poor quality messages guys'll send to you.