Pros
Consistent and pretty secure, which given the recent economy is valuable. The organization as a whole is too big to care about you, but you'll generally be working in a smaller team, so the people you interact with directly do. Expectations are clear and training is very thorough.
Cons
Paying as little as possible is the core of their business model; otherwise, their clients would just hire their own administrative staff. I've never heard of anyone being cheated out of pay, but pay is low and raises are minimal. This is a good place to start and build experience before going to somewhere that pays a living wage, but you have to get pretty lucky to be paid decently here.