Great network of clients to work with
Flexible compensation and benefits - they let you choose what makes the most sense for your situation
They actually care about you as a human being - they'll work with you to find the best client placement based upon your needs
They will support you to achieve your career objectives - they will guide you to needed training, target your next opportunity, etc to in order for you to achieve your goals.
Cons
They are the best staffing firm I've worked with, and I have been in IT over 18 years and worked with many.