I havn't looked at this, but I guess if you allow co-workers to rate each other then all that's going to happen is that the office politics gets transferred onto the site, with a lot of people bitching about each other, which might not be such a great idea. Also if you're rating co-workers from previous companies then old animosities could be perpetuated online.

I'm only cautious because in a couple of places that I've worked at in the past the office politics was occasionally quite toxic, and amplification of that via the internet could have quite serious consequences.