I recomend not using them. Tells the employer that the candidate was too lazy or too busy for their own leg & networking, so how will that person pan out on task.
About every six months I would be contacted by some British based agency, they would claim that they could provide me with all the experts the corporation needed. Which I found odd since they did not know the scope of our work when asked, they would say that can get the best SIA qualified personnel out of the UK.
These agencies are ran by twats, they do not know what the potential clients want, and they will throw anyone they can into a job.
Just get out there, refine your abilities, learn new skills, listen to your elders, raise your personal standards daily, set a long term (maybe three years) goal, and at the end of it all look back and see where you excelled, where you f*cked up, what should change and what you would like to change. Then take a holiday, adjust fire and start all over again.
Just my thoughts though, it has never let me down.