Pros
Very good at selling you to a potential client. But be sure to get the pay you desire upfront. They do have a decent benefits package for contractors, and the recruiters and account managers are generally friendly, and genuine. However in the bean counter room in the back office somewhere I think the recruiters are threatened concerning keeping hourly compensation as low as possible.
Cons
They will first try to undercut your value with a low-ball offer on pay. They pay much lower for seasoned engineers, and have. I have worked for Teksystems on three different occasions, but I have swore them off for good, even though I get calls for open positions form them at least six to eight times per year. However I don't like the low-balling, on one hand and the almost desperate requests made to me to allow my resume to be submitted for positions on the other.