Vantaggi
They always pay you on time, and offer the standard/required medical benefits and retirement benefits (not stellar, but not bad). They do not demand 40 hours every week, but you might hear about it if you took too much time off. Their clients rarely - if ever - approve overtime, so it's unlikely that you will have to work over 40 hours a week. If you do work more than 40 hours, I think - but do not know - that you would get 1.5x your normal pay.
Svantaggi
You are only a warm body placed with their client. As long as you are making money for them, everything is okay. They do not care if your experience with their client is good or bad. If you are experiencing abuse at their client - short of anything litigious - they may sympathize, but otherwise can do nothing and will do nothing. You are a source of money for them, and that's the name of the game. There is no training to build skills, nor do they care if their client is under-employing or over-employing you.