I have a lot of experience with recruiters and most of them have been bad. There are several versions of recruiter.
Some basically get a list of job seekers and a list of job openings, and try to play matchmaker. They are glorified versions of HotJobs.com, and I'm not sure how they get compensated.
Some represent you personally, and will shop your resume around to different firms with which they have relationships. These guys usually require you to pay them a fee either on either a monthly basis, or when they find you a job. I personally wouldn't use this type of service because I think it's dumb to pay for somebody to find me a job. Effectiveness is debatable.
The ones I deal with most often are working with a company to find qualified candidates for a specific position. These guys are compensated by the company when one of the candidates is hired. While this is a good concept, I find the process more annoying than anything. The recruiter seems like a useless middleman. You ask a question, and he calls the company to find the answer. Why don't I just call the company directly? I've never actually taken a position that a recruiter has brought to me, but I have interviewed for several of them.
I've had a lot more luck just doing it the old fashioned way. If you are qualified for a job, you just have to be patient and wait for your resume to get noticed by the hiring manager. The job I have now, it was 3 weeks between when I sent my resume and they made first contact. A month, and 3 interviews, later, and I had the job.