I had two set backs from "recruitment agencies" early on in my career who picked through my CVs and diagnosed: No way Jose. I got exactly the sort of job they said wouldn't be possible for me.
Having said that I now several great headhunters (maybe I like them more because they are now calling me, and hence looverly [wink.]) What LizzyLou said rings quite true and they do encounter candidates who don't bother to read job descriptions when they apply to ads or follow their briefing advice before interviews. I wouldn't let them be my excuse for not pursuing a particular career if they were negative towards me but if one of them takes the time to offer insight into a potential employer, I wouldn't dismiss their advice out of hand.
People don't really like 'informational interviews' in this country but I have actually found headhunters more open to these discussions than potential employers. So, maybe what you could do is approach headhunters/recruiters in this manner: have a clear idea of what sort of job you want, even WITH WHOM (range of companies based on your research), seem like you've got a direction and are going places etc. You may just find that they are willing to engage in discussions like these which could be useful and in the end productive- at least more useful than 'here's my CV, you do all the work now.'
If you've been out of work for a while they might not erally know what to suggest - you may need to get them on side a bit, show them you're really motivated. They will be more inclined to help you.
Early on, I had a headhunter who approached a company on my behalf. It didn't work out and there are risks to this strategy but if the headhunter knows the company it can also be an inroad - then they are motivated to help you becasue they poss get a fee if you get the job.
You need to do your own research for this to work but it is one strategy I have seen people use. It may well make MORE sense to approach the company directly - it all depends on your industry and the access you think the recruiter may have.
In my case I transitioned out of an arts background into finance - lots of questions like: Can you really do this? Yours may be more: Do you really want to do this now. But it's simlar.