Being on free hosting, we do not have the rep mod added. However, even if we were on advanced hosting again, I still wouldn't add this mod, especially after all the abuse issues they had here on FP a few years back with people abusing the rep system in general. That alone would keep me away from that mod.
I think negative reputation is just a way for members to get back at another member. In some ways it can be useful, like ratings, etc. and I can see the benefit of it. However negative reputation itself is just something I won't ever be using.