Vantaggi
If you do good work for their clients, it is easy to reach out again after some time away from temping and pick up jobs right away. They seem to keep you in the system. The company seems run with integrity. I don't know how they treat their regular, full-time employees, but I always felt they were on my side. I only ever worked with supportive account executives. Lastly, the last time I worked with them, I specifically requested short term, temp positions only, rather than long-term or temp-to-hire, and they were able to get me a fair amount of work.
Svantaggi
Obviously, you are dependent on them to help you get work. You may end up in a position at a company that you would otherwise never work for because of their office culture, for example.