I don't have the end all be all answer but my short answer is YES. Basically, they are a headhunter's haven. They are a place where you can get screwed over by a headhunter who doesn't know jack about you or the company for whom they are peddling a job. Also, they seem to be yet another HR scam to get as many resumes as possible and then throw 99% of them away. Now, I'm not saying that all headhunters suck nor am I saying that the job sites are completely worthless. I am, however, saying that you can spend your energy in much wiser ways.
From James Shaw's quasi-blog:
"Finding a job is like waiting for a #10 bus - none come for a long time and then 3 come at once.
It took me 3 months to find a job, and then I found myself with 4 job offers in 2 weeks. Luckily the 3 months wasn't painful - my employer gave me 5 months notice that my office was closing in June, so I was employed the whole time. I know many developers that have been out of work for over 12 months which doesn't bear thinking about."
There has also been some interesting discussion on this matter over at the [jobs] list at ASPAlliance.