I can only address what I've done regarding temp agencies.
My apprentice (starting 4th year with me) worked through a temp agency for @ 1 1/2 years, as I didn't want to have the hassle of taxes, workmen's comp, etc. His cost through the agency was 50% of his agreed scale. (Ex: Apprentice paid $12.50, agency charged me $18.75) For the short term, it worked out well for me. After I knew he was going to work out, I paid his health insurance as well as his apprenticeship schooling (tution, books, fees,etc) Eventually, as he continued to get raises, it was more cost effective for me to employ him myself.
I just hired a young man to try out as an apprentice. Same arrangement (Kid gets $10, agency charges me $15) If he gets hurt on the job, it's on the agency's workmen's comp, not mine. If he doesn't work out, I call the agency and he doesn't come back. He's temp labor, so I'm not required to put him through school. (Actually, he seems to be doing well, but we'll give it a couple of months). I believe this is a win/win situation, especailly regarding workmen's comp. We all know how hard it is to get good help.