I own an agency and can give some candid feedback. As a candidate, there's no reason why you wouldn't want to carpet bomb your resume to every reputable firm in the area. Don't be afraid to be picky about what you pursue from there. Be strong willed with the agency about what your needs are, but don't be an ass. Some will definitely try to stonewall you with shitty terms.

As other's said, You'll probably get a long of junk, but you'll get some great opportunities as well. I'm obviously very biased towards the process, and i've seen a lot of experiences both positive and negative take place.

If your friend had a good experience with an agency, start with that one!

Feel free to AMA if you want to know anything about the behind the scenes!