Personally, I have always found agencies to be a bit of a waste of time. Mainly because operators only usually go to an agency if they are desperate, and that is usually because they can't get anyone to work in that location/ for that money/ etc.
My experience was that after having suddenly become freelance, within about three months I was turning work away with operators calling me directly to find out if I was available. I also found that replies like "No, I'm not available because I'm going sailing/skiing/promised to take Mrs to theatre" don't stop them calling.
The result was that when most of these operators tried to offer me a full time job I turned them down, since who'd want to go back to being at their beck and call?