Two friends of mine used an agency in London and successfully found new partners who they have since married. The agency had a process which required an interview and an upfront fee.. it was quite substantial.. I think about £700 or something like that... I suppose this prevents time wasters joining... then, the agency selects possible matches..
In both cases, I think they settled on the second person they met... it obviously worked for them...
I looked into one of those agencies, can't remember the name now, during quite a long period of being single. (I actually really enjoyed being single, by the way, but it had been about 4 years and I was starting to think I'd quite like not to be!).
As DV says, you have an interview so they build up quite an extensine profile on you and what you're after, and the agency actually puts you in touch with suitable matches rather than you having to trawl through hundreds of unsuitable people online.
I think their success rate was quite high, but it was expensive - There were 3 or 4 different levels of membership (ranging from about £500 to £2000 if I remember).
It's intended for people who are seriously looking for a long term partner. Whereas if it's just for fun and dates with lots of different people, the online option is probably better.
Edited to say: I went for the interview, but didn't sign up in the end as my circumstances changed.
Edited: 11/01/2013 at 14:14