Pros
They have such an established history, people know their name, and they have some very reputable clients that I would not otherwise had the opportunity to work for. The companies you get to work for look great on a resume, and gave me the experience I needed to get the permanent job I needed.
Cons
Each new assignment presents the challenge of being a rookie all over again. You are always on the bottom of the totum pole, and even the lowest level employee has no idea what your skill levels or background are. They assume you are no more skilled than they, and they treat you as such. It can be a humbling experience! Also you get used to one schedule and location, and then you have to switch your whole routine for a new assignment. You have to be flexible.