Pros
At first you can be fooled into thinking there are pros of working with this company as a contractor for specific clients but you will soon learn that there are very few advantages. Apart from actually getting you a job in the first place, that is where their service stops but they will also get you to sign a pre-contract so you can't go anywhere else in negotiations.
Cons
Despite talking a good game about being the people's company and caring about their employees, this is not a true reflection of their actions. They serve the client only and not the employees. You can be harassed, bullied, even physically attacked in a job and they will take the client's side and you'll likely be summoned for a dismissal hearing for the privilege. They also trap their employees into service contracts where you are simply unable to be promoted or increase your salary. I've never felt so unsupported by a company in my 20+ years of experience. If you have another option for a contracting company, go with them.