I want to find the companies that have a vacancy, NOT the bureaus that want to hire you out to a company they keep secret.
I guess it depends on how badly you need work. It's not like the headhunters can keep their customer names secret from you forever, and if things go well you do get a job out of it.
Of course, there may be cultural differences between headhunter firms too. My direct employer is a consulting firm, but we also do contract to hire work and sometimes direct placement. When we bring someone in they go through a screening/techout process, after which we submit them to customers. Usually the contract terms specify that the customer has right to hire after 6 months. We do a lot of business with large companies, and a lot of them prefer to hire employees that way rather than going the traditional route. The customer gets a chance to evaluate the potential employee's performance with no strings attached. If they don't like them they can send them home immediately or at the end of six months or anytime in between, without all of the messy work of firing someone. So in many cases it's a try before you buy.
Of course, it also works that way for the potential employee as well. You get to try out the employer to see how well you like the environment, type of work, the boss, or the hours. If you don't like it you walk away at the end of six months (or sooner if it's that bad) and there's no strings attached.
I have seen some staffing companies that tend to lowball people on salary, and then do a high-pressure pitch to get them to sign on with the customer. Usually that's because the contract is loaded with a big payout on the back end if the prospect hires on, but only has a small margin in the interim. The better companies will set a higher margin for the trial period with no buyout. That way they get paid either way. Most customers are OK with that though, because those are the staffing firms that tend to attract the more qualified candidates.
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CompTIA A+, Network+, Server+, Security+
MCSE:Security 2003
MCITP:Enterprise Administrator