Pros
Kelly has been around longer than many and has "ins" with many established companies. They have always been willing to negotiate a higher hourly rate or salary for me; they don't just pay lip service to advocating for both contractor and company, then server only corporate interests. Kelly seems to hire friendly, more down-to-earth, capable and socially savvy recruiters, in my experience. I've worked with themk, off and on, for twenty years, and no other employment firm has equalled or surpassed them for customer service and ability to find jobs.
Cons
Recruiters are in business for the bucks. You pay for the conveniences you receive. Many contracts never turn into full-time positions, if that's what you seek. It's in Kelly's (or any staffing firm's) interest to keep you on the pay-roll, regardless of feasibility for the long-term. So, don't look to them to be your advocate and keep your eyes and ears open. You're kind of a skilled mercenary, in terms of needging to depend upon yourself, politically and for protection in the workplace. That said, there are benefits to being on conract, especially if you're savvy and keep your skills and your work polished.