One of the employees that worked with me (he's also a friend) went to a recruiting agency that is well known in the CE industry.  The agency did a resume blast (this is when a copy of your resume gets sent out to every one on a mailing list regardless if they have an opening or not) to all of their clients, sent him on 10 interviews, and he got 3 offers.  One he accepted.  Good deal so far.

Well he was a good worker so my company made a counter offer and he accepted and stayed.  3 months later he got let go due to a drastic slowdown.  Then he was in need of a job, not just looking to see what was out there.  The agency wouldn't work with him because he accepted the counter offer.  And literally every engineering firm within 25 miles had seen his resume through the agency and would have to pay a fee to hire him because they had seen his resume through the agency first.   The poor guy ended up accidentally blacklisting himself and had to leave the industry b/c he couldn't find a job.  It all would have been fine if the agency had just said to him "we're going to send your resume to companies X, Y and Z" or had let him know they were sending his resume out to everyone.   

Be sure to interview the agency before letting them have a say in your future.  Here are a few example questions:

-How do they plan on getting your resume into the hands of a hiring manager? 
-Find out if they work only on qualified openings or if they are focused on "proactive recruiting" (sending in a strong resumes if there is an opening or not)?
-What makes their company better than other agencies in representing you?
-Ask for referrals - Ask to talk to candidates that have they have placed before.
-Are you signing or making a verbal agreement with the agency to represent you?
-Is there a time limit on this agreement? Is there a way to get out of it is things go sour?

So when deciding on a recruiting agency to represent you, remember, they are representing your career as well.  They also get paid by placing you (i.e. make a commission/fee from helping you get a job).  You need to make sure that company is focused on your best interest, not their wallets.

There are MANY MANY great recruiters out there that will do a great job.  But there are companies out there who aren't, and you know the saying about a few bad apples ruining the bushel...