If you are using agencies as an employer you can choose them. If you are using them as a jobseeker you can't.
The problem is that agencies act for employers, and many employers will only recruit through agencies (because they don't have enough HR staff to do it themselves). So if you refuse to deal with particular agencies as a job seeker you are effectively taking yourself out of visibility of the companies they represent. Frustrating but true!
About the only thing that seems to work when dealing with them is to put simple key words at the top of your CV, the entire job search process seems to be based on this now.
And don't believe anything they tell you about any prospective job, all they care about is getting you to interview - only the company you are going to work for will be honest about job role, salary, perks etc.
One thing I have (I hope) now learnt by bitter experience is not to get talked into going to interviews which do not seem right for you. A few years ago I spent a fortune in rail fares going to two interviews which seemed unlikely - and both of which became very embarrassing when it became clear I was not at all what they were looking for. Ok, the companies involved should have been able to tell from my CV that I was not what they wanted, but presumably the recruiters involved had told some very tall tales on my behalf.
Andy Millar CEng MIET CMgr MCMIhttp://www.linkedin.com/in/millarandy
"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert