Depends. My ex worked at a Fortune 500 company through an Engineering agency (Kelly Services) for over a year..and there were guys working there through the agency for more than 10 years. They didn't have as good of benefits as the people who worked FOR the company directly, but htey were content with what they got.

Agencies like Sesame and Manpower (dunno if you have htem up there) you'll only work at one job for a few weeks at a time, if that, then get moved to something else.

It depends on what you're looking for and what agency you go through.