Well, I met my girlfriend on match.com, and we've been together for almost four years now. I do think that online dating makes a lot more sense than going to a bar and hoping to meet someone with whom you might be compatible. By letting prospective partners know certain things about you right up front, you might get fewer dates, but the ones you do have should be better matches.
I know a couple of other people that have used online dating sites. They've all tried match.com, which has worked out OK; all of them have had long-term relationships that resulted from that site (though some of them are now over). No one I know has had success with eHarmony. My girlfriend was on it for a little while, and said that they don't let you choose any partner you might want. Instead, they give you a list of people with whom they think you might be compatible, and if you don't like any of them, too bad, because they won't give you any more options. If that's the case, you would just have to wait for someone new to join the site with a similar personality profile, which can take months. The one date she had through eHarmony was an unmitigated disaster. When you factor in the price, eHarmony is even less appealing.
One of my friends cycles through the dating sites every few months (goes on one for a little while, then tries a different site for a few months, then goes on hiatus for a month or two, then goes back to another dating site). He figures that this gives him exposure to a lot more people than if he just restricted himself to one site, but keeps the cost down. Some people will be on multiple sites at the same time, but that can get pretty expensive.