They're drop you like a fly once you ask for a competative salary or consulting rate. If you're a Software Engineer, no matter how many years and experience you have, they're gonna treat you like a Jr. Developer and try to get you into a job at $75/hr.
They'll tell you "we are moving fast on this position" because they want to lock you into an uncompetitive rate, and don't want to give you any chance of debating the rate.
Don't waste your time. If you are just average talent, and $75/hr works for you, more power to you. But if you mention anything normal, or even market rate, they ditch you right after the first conversation.
They're for placing amateurs for monolithic companies who do not value code quality, only value cheap and dirty get it out the door cultures. They're not interested in placing real professionals.
