This is a great question and I applaud you for asking it. Although I understand Nicholas's view the title is designed to elicit some constructive feedback for the industry which in my opinion doesn't do enough naval gazing.
In my 25 years in roles as a GM, Senior HR, Organisational Development and start up roles I've engaged well over a hundred or so Recruiters and Search Execs and I've found on balance if there is an opportunity to tick and flick a resume and make a quick fee it is almost always taken. When I'm paying 18%+ fee I want candidates that have been properly screened.
More specifically I often find the following:
Letting the investor tell me (founder, ceo) which headhunter firm to engage - behind-the-scenes the investor total "owned aka influenced" the headhunter because, unknowingly, they used the same headhunter firm for every one of their investments.
I've been blessed with a number of great recruiters.
This applies to being the hiring manager and the one being recruited.
But I had one that stood out as being terrible. He hired me as a manger in a company and I was forced to use him for a number of months before convincing the company he was a bad choice. He was already on contract, so it took some work getting rid of him. His rate was insanely low (I think around 1k for a successful hire), but I eventually convinced the company he wasn't worth it even at that cost.
He just fired resumes to me with little to no screening, so I ended up doing all the filtering. Almost every candidate who walked in said something bad about the initial meeting with him. I remember when he introduced me to the opportunity, I asked him about the team, culture, etc. What he told me was so far from the reality I uncovered at the first interview. Things like "the average age of the company is quite old" while I found the team to be quite young when I walked in the door. The worst thing he did was re-send a candidate I had already rejected. All the guy did was slightly tweak his name "Phil" to "Philip" and he missed it. I noticed the CV the moment it came across and checked my records. Clearly he didn't have any kind of CRM or even Excel to keep track of candidates.
Just goes to show...often you get what you pay for.