Pros
You're exposed to the latest web technologies. There are many opportunities to learn new systems and the latest and greatest coding technologies (if you're a developer). Developers have the best chance of being converted from contractor to employee.
Cons
They bring in most people as contractors, at which point you're used and abused. Employees are told to give the crappier assignments to the contractors, but the contractors are just happy to have a job in this economy so they take it. Majority of workers are from India and will work endless hours, as most don't have family here. The Indians are seen as better workers because they don't have the same constraints as the American workers who have families and want a work-life balance. They'll work night and day, weekends and holidays. Most of my Indian co-workers were wonderful people, but it's difficult to have a positive view of a company that mostly hires foreign workers when so many Americans are jobless.