Ventajas
They are a necessary evil if you are contracting at a company that uses this contingent workforce agency.
Desventajas
Working as a contractor through Workforce Logic was a horrible experience. Their employees aren't knowledgeable about any of the company policies and it is a nightmare getting information about pay, benefits, and the like. As their client is the company where you are working, they have absolutely NO interest in doing anything for the contractors they are hiring out. Do everything you can to avoid signing up for COBRA after your contract ends. They use an outsouring vendor, eCOBRA, to service their ex-contractors and eCOBRA doesn't forward premiums onto insurance promptly (leaving you to pay out-of-pocket). I understand their policies have changed and they charge an even larger fee to employers (which effectively comes out of your pay), even when they have done nothing to recruit.