Hello,
I asked a somewhat similar question here a while back and got a few mixed responses.
As far as I can see, the recruitment industry is unregulated and a lot of firms have "popped up" in recent times. Calls not being returned, claims of agencies making up jobs that don't exist and advertising them to collect CVs, or recruiters just looking for the "low hanging fruit" etc. are widespread. I have also noticed some employers purposely excluding recruitment agents when they advertise a job directly, so they must have their reasons. That said, not all recruitment agencies are bad, no more than not all taxi drivers, bankers etc. are bad. Personally, I would look towards the long term established firms - particularly those recognised for recruiting in the industries your interested in.
I would also suggset you get set up on LinkedIn - there are a lot of jobs advertised there by both employers and recruiters. Where possible, I would deal direct with the employeer.