Overall, my experience has generally been positive; there are a lot of good positions that I wouldn't see otherwise. Years ago, a recruiter placed me in a position that really kicked off my career in a good direction.

Having said that, I get a call every month or so from a recruiter who saw my resume on Monster, but didn't seem to read it. I get calls for positions that I'm not qualified for, which is a huge waste of my time. The recruiters who actually read my resume and suggest positions for which I am qualified are worth my time.

Each agency specializes in a field - do your research if you are approaching an agency.