Considering agency per diem work locally. What do I need to know/expect?

I'm considering working per diem for NCStaffing (a local and travel nurse staffing agency). The pay is good and you get paid that same or next day it seems. There are some extra shifts available to me at my work but not always as I need them and I have to wait to get paid. So what should I know before I sign up and what can I expect? I will still be working my weekender position and I really just want 1-2 shifts a month. I've been a RN for two years, have ACLS too.