I only have a single direct experience with a headhunter agency, and they were actually quite nice.
In the end I basically just gave them my resume, told them what jobs they forwarded sounded interesting (a handfull each week) and then went to the interviews that came up.
They also managed to negotiate a higher salary for me (due to their fee being based on my first year salary).
So at least in that case they did work “for me”
time for what if xkcd: “what if every human and animal on the planet farted at the same time?”