I only ever met had a good experience with one agency, and I didn't manage to get a job through them (well, what do you expect, A-Levels + virtually no commercial experience). (I think I did get a couple of phone interviews via them).
The good side being that I randomly got a nice calendar from them about a year later. It'd be interesting to see if they're actually any good when you've got skillz. -CT The URL is: http://www.a1-cliveden.co.uk/ Least, I think that's them, this was back in '98, so things might have changed a bit since then.