I think they, along with other internet social venues, are the wave of the future...heck, the present...especially for anyone who doesn't regularly meet scads of new eligibles every day. I don't know how it is over there, but in the States, I swear they're becoming the only sensible way to meet people.
The voice of common sense and tradition will say that they're scary, that anyone might be anyone, that "you never know who you'll meet." But if you take precautions, use common sense, and share details and meeting times with others, you'll enjoy the benefits of knowing straight off if they're a smoker/non-smoker, want kids, are married, have allergenic cats, have exactly YOUR wacky sense of humor, live within range, etc. - all the stuff covered in people's online profiles. This is WAY more than you usually know about someone you meet at a party, or even at work or school or church - and you get to find it out BEFORE you even contact them. Many people who meet traditionally don't find out this sort of stuff until they've been dating for a year!
And just as in person, some people lie, but most people don't. Some dating sites are fee-based and have identity checking services to help protect their users. You definitely DO have to be careful. Then again, so do you in "real life." And on the Internet, at least, you're less likely to fall victim to someone's charm. Charm does NOT a permanent relationship make, but it can surely waylay you for a long time...!
I met my significant other through a dating site and know many people who did the same. It's like self-publishing in today's technology-enabled world - not tawdry anymore.
The big challenge with online dating is actually figuring out HOW to find the one who's right for YOU. How to recognize that person in writing and photos or videos, and how to present yourself to candidates. It's a skillset in and of itself!