In my experience, they're fairly up-front about stuff. The thing is that, as far as I know, they on'y get money when someone gets hired, and sending people out who are not thei right person for the job only makes them look bad in the eyes of the prospective employers, so they're not too willing to do that. That's not to say that they won't make mistakes, especially since they don't know the ins and outs of what we do for a living, but most of the jobs I saw had fairly explicit requirements. Most of the time no one person is going to meet them all; they're looking for the perfect person, who doesn't exist, but in this job market, they can get much closer.

In my experience, you'll call and talk to someone at the recruiters' place. Once they find a job that looks like a match, sometimes initially, they'll call you in to interview with them, so that they can get a better idea of who you are. It's not a technical interview -- more of a get-to-know-you -- so don't be nervous and do make sure to be personable and distinctive so that they'll remember you.

In my experience, they don't have access to many, if any, jobs that aren't externally posted, but employers do respond better to those agencies because they've done some initial screening to find appropriate candidates instead of getting resumes directly from every schmoe who's out of work, especially in these times of high unemployment, since the population of schmoes has increased dramatically. To illustrate, as part of my unemployment benefits from the state, I had to apply to at least two different employers each week. I never heard any responses from any of those contacts at all.

It's also apparently bad form to get one's resume to an employer twice (probably due to deciding which agency's fee to pay, or none, if you sent it in directly), so try to avoid doing that. Most reputable agencies will ask explicitly before sending your resume anywhere, each time. That's usually as part of a call to see if you have particular skills the employer wants.

Of course, this is all just my experience, both from getting jobs and hiring a few people here and there; it's far from comprehensive.