My first two positions were through a job agency, and both experiences were pleasurable. When I first got out of high school, I thought it would be a matter of looking up in the paper, and finding a job. After so many interviews, and 2 months of doing it the hard way, I eventually gave in and put my CV through an agency that my brother had always used. One of the agents rang me up. The next day I went in and had an interview did the standard typing test, and then she submitted my resume to a company that was looking for a junior secretary. Their requirements were for a young person, with a business college diploma, seeking employment as a first job. I did not have a college diploma, but I was young, keen and well presented. The agent had my name submitted, two interviews later, I got the job. All in all, it only took 2 weeks to get that job, and I stayed with the company for 2 years.
I used another agent, for my next job, and within 2 weeks had another job.
My current position I did not use a "head hunter" (we call agents head hunters in Australia), to find it, I just found the position in the City Weekly, the Sydney magazine that lists jobs etc. I also was using two job agencies, but had a bad experience with both.
However I would never judge those two bad experiences as entirety of my opinion on agencies, as that would be generalising. My previous two experiences prior to those two bad experiences were fantastic, and I think in general it pays to do your homework. Most agents specialise in industries, that is some specialise in financial services (ie. insurance, banking, etc). Again its about doing your homework, in my opinion. Its sort of like real estate, shopping around for the best property on the market, whereas you are shopping around for the best job. Its always important to advise your agent your expectations, that way they know what you want and will not just put your resume forward to every Joe Blogg Pty Ltd out there. I know there are some rogue agents out there whom are only interested in cashing in on the bonus that they get for doing the job, and have no interest in the person they marketing for the position.
Again I will say do your homework.