Should I be wary of full-time staffing agencies?

I have a BA from NYU and I graduated with a masters in statistics from Columbia this past May yet I cannot land a job. I am looking for an entry level position in finance--yst, marketing, administrative...I'll do anything. I live in the NYC area, but I cannot get an interview. So I am thinking of going to a staffing agency that places people in full-time positions. I have never done this before...are there things I should look out for? ARe they reliable?