Personally I have had most success working with agencies who have an office in the area I want to work in. I make sure I meet the agent who is going to be "selling" me in person & build up a rapport with them. If you come across well, they will be happier to put you forward and talk to employers about you as a person.
Some of the on-line agencies employ data mining techniques to sift words from your CV and make statistical matches with the job spec. Having seen job specs for jobs I know, that doesn't fill me with hope.
I used to do a lot of organisational change work & when recruiting, I used to get loads of CV's for ITIL change managers - not the same thing at all, but the words matched!
I'd say use the on-line sites to see if they have the right sort of roles or work in the right area, and then ring up & see how the people in the office work. Make your decision on how that converstaion goes.
Cheers
