From the website:

Lots of social networking sites let you gather "testimonials" from people you have worked with. But everybody knows that these people are your friends and will only say positive things. Coworkers.com gives you real, balanced, detailed feedback about your professional reputation, and lets you share the truth about your coworkers - positive, negative, and in between. And because you can remain anonymous, you need not fear retribution. Coworkers.com acts as a central repository of peer review from throughout your career, and it is trusted by employers as a legitimate source of information.