Every year I got glowing praise from the client - I solved the problems they had faster than their in house guys did, I delivered on time with the lowest defects of any contractor across all firms that worked at the client, and was a domain expert in one of the client's systems after about six months there.
Do you know what the consulting company offered me as a raise? $150 on top of my base salary. Not $150 a week or month - $150 total. When my consulting company reviews came back, despite high praise from the client, I wasn't enough of a member of the consulting company's team because 1) I didn't hang out with only consultants from the company, 2) I didn't attend drinking events every week with consultants from that company, and 3) I was too specialized on what the client did, so how were they going to move me to another client for more money?
I stuck around for another year and then the client negotiated hiring me on directly.
I will starve before I work as a consultant at someone else's consultancy. I'll start my own one-man firm first.