I've done it in the past, and I'll probably wind up doing it again. I'm too shy to approach guys in public, and when one approaches me, I tend to freeze up and get all tongue-tied, and they seem to think I'm not interested, so they walk away. So...yeah. I can open up easier online. Plus I have an easier time editing myself when I'm typing than when I'm speaking.
It REALLY depends on where you're looking, though. Some free sites are just full of people looking for hookups, and it's really hard to find a serious relationship. And the whole matchmaking program thing...meh. I'm not convinced it's worth the money. I was on one of those sites, paid, for a year. Which included three months of no internet access...so I felt a little gypped. At least there are all kinds of niche dating sites now, so you can cruise through a pool of Christians, geeks, BBWs, or whatever else catches your fancy.
If you're smart and careful (ie meeting in public places) and honest with yourself (ie knowing what you want in a partner, not just looking for pictures of hot guys/girls/whatever) then you can have success.