I’d say 1 out of 10 headhunters is worth even dealing with in my experience. A good litmus test is to see if they even acknowledge receipt of your resume. I was placed at my current position by an excellent recruiter who was a pleasure to deal with, but by in large my experience with them has been negative. No responsiveness, no follow-up, cancelling interviews at the last minute, etc. Had one guy tell me that it was a done-deal that I’d be placed and that the interview would just be a formality, after which he established radio silence and I never heard from him again.
Much like field sales, I see this industry shriveling up in the next 5-10 years; still players in the game but far fewer. The quality ones will be the ones that survive.