Avantages
I've been with CTG as a software developer for 10 years. The company is reliable on paychecks and will do their best to place you. I rarely interact with CTG, which isn't a bad thing. I work at IBM and my paycheck comes in seamlessly. CTG sends out monthly newsletters and quarterly letters. Overall, you feel like you work at your client site, but you know CTG is always there for you.
Inconvénients
CTG doesn't match 401K, they offer bad health care, and they will not cover your spouse for health care. They'll cover you and your kids, but not a spouse. They also don't have a large client base. It's really just IBM, which is fine if you want to work at IBM. Once you agree to your hourly rate, that's your rate for years. There is no pay raise and you can't negotiate for one.