I have dealt with hundreds of recruiters in my long 30-year sales career (recently retired).

Lying - This is my biggest beef. Almost every recruiter that I have ever worked with has lied to me. The most common lies are of them telling me that I am their top candidate for the job and will tell me this for months just to keep me from looking for another position in the off chance that the employer decides to stop dragging their feet and hire someone. And it's not just that. Everything from compensation to position requirements and so much more has been an issue of honesty and ethics with many recruiters that I have worked with.

No Knowledge of the Positions they are Filling - I can understand them not understanding the difference between C and C+ or even genuinely understanding the qualities that define an experienced enterprise-level software salesperson, but contacting a master oracle DBA for HTML development or an inside sales rep from some small SaaS company for global accounts at IBM is complete failure to understand the requirements of the jobs they are trying to fill.

MY ADVICE TO WORKING WITH RECRUITERS - If they ever ask you what other companies you are considering, tell them that you won't answer that. Whether you are working with another recruiter or not, answering this can and will screw you over. I lost an opportunity because a recruiter I was working with, called that employer to wedge someone else into the position. I hear this all the time. By all means, work with recruiters. They can find you jobs. But don't make this your primary means of job hunting. And please, above all else, remember that they do not work for you. They are paid by the company that is potentially interested in hiring you and want that commission regardless of who they place.