Pros
Access to training such as SkillPort Depending on the area of the country there are some good regional offices, my experience does not mesh with what I know some others have experienced, so talk to people in your area to determine if yours is a good office.
Cons
They do not follow through on their financial commitments, and when you take on a higher bill rate with more responsibility they tell you that you are salaried and do not get an increase other than at normal cycles. Even at those normal cycles you will get little, if anything. I was also incredibly discouraged by their lack of development opportunities. We had to band together on our own, without their assistance to create a community at the client location where we could share information. They sell the idea of their "bench" as a benefit, but they do no proactively search for positions when there are many staff coming off assignment (not sure of the cause, maybe understaffed in that area) but if you aren't constantly calling them to make sure your name is at the top of the list then you will likely end up being laid off after a few weeks.